WASHINGTON — Just days after an American civilian contractor was killed in Iraq in a missile attack by Iranian-backed militias in December 2019, President Donald J. Trump responded by ordering a drone strike that killed a top Iranian general.

After an American civilian contractor was killed and six other Americans injured Thursday in northeastern Syria by a drone that US officials said was of “Iranian origin,” President Biden’s response has been more restrained thus far.

Two US F-15E fighter jets retaliated Thursday by conducting airstrikes against militant sites linked to Iran’s Islamic Revolutionary Guard Corps. This prompted Iran-backed militias to launch a missile and drone attack on Friday, wounding another American.

A senior US official said US warplanes were preparing to launch a second round of retaliatory strikes late Friday night, but the White House held back.

On Monday, Biden administration officials said, after a weekend devoid of new militia attacks and bad weather in eastern Syria that would have made targeting rebels more difficult, the military is ready to respond to any new threats to American personnel.

But they also seemed eager to move forward, avoiding escalating back-and-forth strikes into a broader war with Iran and its proxies, and staying focused on the broader task of helping root out pockets of Islamic State militants still staging guerrilla attacks. in the area.

“We will do what we need to do quickly and boldly to protect our people and our facilities in Syria,” John F. Kirby, a spokesman for the National Security Council, told reporters on Monday. We will not back down from continuing to pursue this network in Syria. “

America still has more than 900 soldiers and hundreds of contractors in Syria, working with Kurdish fighters to ensure that the Islamic State, which was ostensibly defeated in 2019 after five years of ruin in Iraq and Syria, does not re-emerge.

In the past year alone, Iran-backed militias have launched dozens of attacks on or near bases where US forces are stationed.

Biden sought to allay fears on Friday that tit-for-tat strikes between the United States and armed groups could spiral out of control, while at the same time warning Tehran to keep its proxies in line.

“Make no mistake, the United States does not, I stress, seek conflict with Iran,” Mr. Biden said in Ottawa, where he was on a state visit. But be prepared that we will act aggressively to protect our people. That’s exactly what happened last night.”

Some analysts have raised concerns that the rival air strikes threaten to derail diplomatic efforts to calm tensions across the Middle East, including a recent agreement between rivals Iran and Saudi Arabia to end years of turmoil. The fight also comes as the administration grapples with how to help Ukraine in its war with Russia and how to counter a rising China.

But some military analysts and former Defense Department officials said on Monday that the administration’s retaliatory strikes would not deter Iran or its proxies and that the White House needed to step up retaliation.

said Michael B. Mulroy, former official in charge of Middle East policy at the Pentagon: “The current policy of ‘proportional response’ has not put an end to these gratuitous attacks. ‘If you’re going to strike, strike hard.’”

This is what Trump did on January 3, 2020, when he authorized the attack on the Iranian leader. Major General Qassem Soleimani, who commanded the Islamic Revolutionary Guard Corps’ powerful Quds Force, was killed along with several officials from Tehran-backed Iraqi militias when a US MQ-9 Reaper drone fired missiles at a convoy leaving the airport. in Baghdad.

The strike was a dramatic escalation of Mr. Trump’s growing confrontation with Tehran, which began with the death of an American contractor in Iraq in December 2019.

But this strike did not deter or even temporarily halt attacks on American personnel in Syria and Iraq, as Secretary of State Mike Pompeo and other Trump administration officials had predicted. Indeed, two months later, two Americans and another member of the coalition were killed in a missile attack on a base in Iraq.

Thursday’s initial attack came as US forces in northeastern Syria were on high alert after 78 attacks by Iranian-backed militias since January 2021. But a self-destructing drone managed to strike a coalition base anyway, killing the contractor. American civilian – wagon. An unidentified mechanic – and six other Americans were injured.

Two US officials said Friday that the base’s main air defense system was “not fully operational” at the time, raising questions about whether the attackers discovered and exploited the vulnerability or if they sent the drone around then, two officials said. Two Americans. To people who spoke on condition of anonymity to discuss the investigation.

brig. Pentagon spokesman Gen. Patrick S. Rader said on Friday that the air defense radar is operational, but declined to discuss any other details of the system, citing operational security and an investigation by the Army’s Central Command.

A senior US official said on Monday that the main radar of the Avenger missile defense system at the base, called RLZ, is not operational due to maintenance problems, and that a backup radar failed to detect the incoming drone that crashed into a maintenance facility.

About 13 hours after the drone attack, US warplanes bombed three targets in eastern Syria used by Iranian-backed militias. In response, US officials said the militia launched three missile or drone attacks on other US bases in eastern Syria, wounding another American.

The Army Central Command said in a statement on Friday that one of the missiles missed a US facility, called Green Village, by three miles, but hit a home, causing significant damage and injuring two women and two children.

For now, officials said, the department has not responded back.