American intelligence analysts and officials said on Friday that they expected Iran to strike multiple targets inside Israel within the next few days in retaliation for an Israeli bombing in the Syrian capital on April 1 that killed several senior Iranian commanders.

The United States, Israel’s pre-eminent ally, has military forces in several places across the Middle East. But Iran is not expected to target them in order to avoid a direct conflict with the United States, according to U.S. and Iranian officials who spoke anonymously about intelligence gathered on the expected attacks, which they were not authorized to discuss publicly.

Any Iranian strike inside Israel would be a watershed moment in the decades of hostilities between the two nations that would most likely open a volatile new chapter in the region. Israel and Iran do not maintain any direct channels of communication, making the chances far greater that each side could misread the other’s intentions. And an Iranian attack would heighten the risk of a wider conflict that could drag in multiple countries, including the United States.

In remarks to reporters on Friday, President Biden said that he expected a military attack against Israel “sooner rather than later,” and that his message to Iran was “don’t.”

“We are devoted to the defense of Israel,” he added. “We will support Israel. We will help defend Israel and Iran will not succeed.”

In anticipation of an Iranian strike, several countries, including the United States, have issued new guidelines to their citizens for travel in Israel and the surrounding region. The Israeli military said its forces were on high alert.

The U.S. State Department barred its employees on Thursday from traveling to large parts of Israel, the first time the U.S. government has restricted its employees’ movement this way since the war in Gaza began more than six months ago.

On Thursday, Britain told its citizens that they “should consider leaving” Israel and the Palestinian territories “if it is safe to do so.” On Friday, India told its citizens “not to travel to Iran or Israel till further notice.” And France advised people not to travel to Israel, Iran or Lebanon, and evacuated the families of French diplomats from Iran.

Details about Iran’s potential attack on Israel are closely guarded, but American and Israeli officials have assessed that it might involve drones and missiles. Iran has the largest arsenal of ballistic missiles and drones in the Middle East, including cruise missiles and anti-ship missiles, experts say, as well as short-range and long-range ballistic missiles with ranges up to 2,000 kilometers (about 1,250 miles).

Iran also has a large inventory of drones that have a range of about 1,200 to 1,550 miles and are capable of flying low to evade radar.

The exact form an attack on Israel might take, what kinds of targets would be involved and the precise timing all remain unclear.

The top American military commander for the Middle East, Gen. Michael E. Kurilla, traveled to Israel this week to coordinate a response should Iran attack, U.S. officials said.

“Our enemies think that they will divide Israel and the United States,” the Israeli defense minister, Yoav Gallant, said in a statement on Friday, after meeting with General Kurilla. “They are connecting us and are strengthening the relationship between us.”

If Iran attacks, he added, “we will know how to respond.”

On Thursday, the Israeli military’s chief spokesman, Rear Adm. Daniel Hagari, said the armed forces were “highly alert and prepared” for any action from Iran.

Iran has publicly and repeatedly vowed revenge for the April 1 airstrike on its embassy complex in the Syrian capital, Damascus, which killed three generals and four officers from its elite Quds Force, an arm of the Islamic Revolutionary Guards Corps.

But analysts say Iranian leaders want to calibrate their response so that it is big enough to send a message at home and abroad that Iran is not impotent in the face of conflict, but not so big that it spirals into a full-fledged war with Israel or draws an American attack.

In the first months of the war between Israel and Hamas in the Gaza Strip, Iran-backed militias regularly attacked U.S. troops in Iraq, Syria and Jordan. But after a drone strike killed three Americans in Jordan in January and the United States launched retaliatory strikes, Iran stopped the attacks by its proxies, fearing a more powerful U.S. response.

Despite the clashes and hostile rhetoric, both Iranian and U.S. leaders have made it clear they want to avoid an all-out war.

John F. Kirby, the White House’s national security spokesman, told reporters on Friday, “We are certainly mindful of a very public and what we consider to be a very credible threat made by Iran in terms of potential attacks on Israel, and that we are in constant communication with our Israeli counterparts about making sure that they can defend themselves against those kinds of attacks.”

How Israel would respond to an Iranian attack on its soil is unclear. The Israeli military “continues to monitor closely what is happening in Iran and different arenas,” Herzi Halevi, chief of the Israeli general staff, said in a statement on Friday. He added, “Our forces are prepared and ready at all times and for any scenario.”

Iran believes it can generate international support for a retaliatory strike by focusing attention on the attack against its embassy complex and arguing that it was merely defending itself, the Iranian officials said.

International law generally treats embassies and consulates as exempt from attacks. But Israeli officials have argued that the building they destroyed was diplomatic in name only, and was being used as a Revolutionary Guards base, as evidenced by the high-level commanders who were meeting there when they were killed.

A strategist for the Revolutionary Guards said Iran wanted to take advantage of the widening rift between Prime Minister Benjamin Netanyahu of Israel and Mr. Biden over Israel’s conduct of the war against Hamas — and not unite them in hostility to Iran.

The Biden administration has not only criticized the level of death and destruction wrought by Israeli forces in Gaza, it has also voiced fears that increased clashes across Israel’s northern borders, primarily with Iranian proxies like Hezbollah, could escalate into a broader regional war.

In an apparent response to international pressure, including from the United States, to do more to alleviate the hunger and deprivation produced by the war in Gaza, the Israeli military said on Friday that it had begun allowing humanitarian aid trucks to enter northern Gaza through a new crossing.

The military did not specify the location of the new crossing, and it remained unclear how many trucks had crossed, what aid agency they belonged to and when the crossing might be open for wider use.

Jamie McGoldrick, a top U.N. relief official in Jerusalem, said that U.N. officials planned to head to the crossing on Saturday to examine it. He said the crossing would be a significant improvement “if it can go to scale and is not temporary.”

After Israeli strikes killed seven aid workers on April 1, Mr. Biden told Mr. Netanyahu by phone that the United States could withhold military support for Israel unless it did more to protect civilians and ensure adequate supplies for Palestinian civilians.

Mr. Gallant, Israel’s defense minister, pledged on Wednesday to “flood Gaza with aid” and said he expected to ultimately see 500 relief trucks entering the enclave on a daily basis. U.N. figures show that an average of about 110 aid trucks have entered Gaza daily since the war began on Oct. 7.

Mr. Gallant also said that Israel would soon open the port of Ashdod, an Israeli city north of Gaza, to accept aid shipments, without providing a time frame.