"They are flush with jihadi recruits, which are coming into Syria, and we think they are sending a number of them into Iraq," a senior administration official told reporters.

In 2011 and 2012, suicide bombings in Iraq averaged 5 to 10 a month. But over the past 90 days, the number has approached about 30 a month, the official added.

Concerns that the chaos in Syria may destabilize Iraq were at the top of the agenda on Thursday when Secretary of State John Kerry met with Hoshyar Zebari, Iraq's foreign minister, for consultations on security, political and economic issues.

"Sunni and Shia extremists on both sides of the sectarian divide throughout the region have an ability to be able to threaten Iraq's stability if they're not checked," Mr. Kerry said.

The leader of Al Qaeda's Iraq affiliate, Abu Bakr al-Baghdadi, and other senior members of the group are operating from Syria, according to the State Department. "Al Qaeda, as we have seen, has launched a horrific series of assaults on innocent Iraqis," Mr. Kerry added, referring to an attack on Sunday that killed more than 60 people during the holiday that marks the end of Ramadan.

Hours before Mr. Kerry spoke, another string of car bombs were detonated across Baghdad during the morning commute, killing at least 30 people and wounding more than 60. Among the targets were a hospital and a cafe. Immediately after the bombings, the army and the police tightened security across the capital, bringing traffic to a halt in many areas.

"We have seen the new violence or terrorist attacks by Al Qaeda more frequently, and it has cost many, many lives," Mr. Zebari said.

Throughout the Iraq war, Syria was a gateway for Sunni suicide bombers who sought to undermine the Shiite-led government and to attack American troops. When the last of the United States forces left Iraq at the end of 2011, Obama administration officials said they believed that the country would be able to cope with terrorist attacks largely on its own.

But the upheaval in Syria, which has attracted a stream of Sunni extremists, has called that assumption into question.

With none of its own military forces in Iraq, the ability of the United States to train and help the Iraqis fight Al Qaeda is limited. But the United States is trying to assist on several fronts, including the sharing of intelligence to help the Iraqis uncover terrorist cells and avoid indiscriminate wide-scale dragnets that fuel Sunni resentment. During the war, Iraq's counterterrorism forces depended heavily on American military intelligence, which used sophisticated reconnaissance systems and had the ability to locate terrorist threats by intercepting communications.

The Obama administration has also notified Congress in recent weeks that it plans to sell Iraq more than $4 billion in arms, including Stryker armored vehicles and other military systems that are suited for internal security operations.

The country's prime minister, Nuri Kamal al-Maliki, has pressed the United States to sell Apache attack helicopters to his government. The Obama administration has been considering a number of options, leasing some Apaches to the Iraqis that they could field quickly. Some American lawmakers have been anxious about providing the helicopters for fear they might be used by Mr. Maliki to intimidate political opponents.

"They certainly would help with internal security, but they can also be used to crack down on the population." said Nora Bensahel, a senior fellow at the Center for a New American Security. "The more the terrorist threat increases, the more likely the sale is to go through."

The Obama administration has also been trying to broker a meeting between the Maliki government and the moderate Syrian opposition. The United States hopes such a meeting will ease the Iraq prime minister's concerns that if President Bashar al-Assad of Syria is ousted, his government will be replaced by a hostile Sunni-dominated government on Iraq's doorstep. It is also hoped that such a meeting would encourage Iraq to be more cooperative in pressing for a transition to a post-Assad administration.

As he did during his visit to Baghdad in March, Mr. Kerry urged the Iraqis to inspect Iranian flights that have been flying weapons to Damascus through Iraqi airspace.

Those deliveries have strengthened Mr. Assad's military position and, critics say, have been quietly tolerated by the Maliki government.

Mr. Zebari insisted that Iraq is not taking sides in the Syria crisis. In a joint statement issued on Thursday, Iraq restated its commitment to prevent "the transit of weapons through its territory."

"There has been some progress in this area since my visit to Iraq in March," Mr. Kerry said. "There is very significant progress yet to be made."

Michael R. Gordon reported from Washington, and Tim Arango from Baghdad.