At a Pentagon briefing Monday afternoon, Lt. Gen. William Mayville detailed humanitarian aid and airstrikes aimed at ISIL forces on Iraq's Sinjar Mountain. (Aug. 11)

A series of U.S. airstrikes since last week has slowed the operational tempo of the Islamic State, the militant group that has seized large parts of northern Iraq, but is unlikely to substantially weaken the group, the Pentagon said on Monday.

"We assess that U.S. airstrikes in northern Iraq have slowed ISIL's operational tempo and temporarily disrupted their advances toward the province of Arbil," which includes the capital of Iraq's semiautonomous Kurdish region, said Army Lieutenant General William Mayville Jr., a senior Pentagon official.

"However, these strikes are unlikely to affect ISIL's overall capabilities or its operations in other areas of Iraq and Syria," Mayville told reporters at the Pentagon.

The Obama administration late last week said it would conduct strikes to protect U.S. personnel in Arbil from the militant group, which has gained strength during the war in neighboring Syria, and to ensure that northern Iraq's minority Yazidis were not subject to systematic violence at the hands of the fundamentalist Sunni Muslim militants.

The 15 airstrikes carried out so far are the first direct U.S. military action in Iraq since the Obama administration completed its withdrawal of U.S. troops at the end of 2011, hoping to mark an end to the long, bloody U.S. military involvement in the country.

Mayville said the Islamic State, which swept into northern Iraq in a stunning advance in June, remained potent.

The group "remains focused on securing and gaining additional territory throughout Iraq and will sustain its attacks against Iraqi and Kurdish security forces and their positions, as well as target Yazidis, Christians, and other minorities," he said.

U.S. directly arming Kurdish forces

In addition to the air strikes, the U.S. now is directly supplying weapons to Peshmerga fighters from Iraq's Kurdish region, U.S. government sources said today.

The weapons shipments to Arbil, capital of Iraqi Kurdistan, come as Kurdish fighters struggle to stem advances by militants from the Islamic State, an al Qaeda offshoot.

U.S. government sources said the weapons were supplied by the Central Intelligence Agency but that the Department of Defense may soon start arming the Kurdish fighters, who regained control of two strategic towns in northern Iraq on Sunday with help from U.S. airstrikes.

Weapons have also been shipped in three deliveries from the Iraqi government in Baghdad to Arbil, consisting mostly of AK-47 assault rifles and ammunition, the U.S. officials said, declining to specify when they began.

Until recently, the U.S. government refrained from directly supplying weapons to the Kurds, leaving that to the Shi'ite-dominated Iraqi government despite Kurdish complaints that Baghdad had deprived them of weapons and financial support.

President Barack Obama has faced growing criticism for being reluctant or too slow to intervene in thorny foreign policy issues which have piled up under his watch, including the dramatic rise of the Islamic State, which has seized control of large swathes of land in the north and west of OPEC member Iraq.

A senior U.S. defense official acknowledged that the U.S. was providing arms and ammunition needed by the Kurds but said it was not coming from the Department of Defense.

Just last week Washington launched its first military action in Iraq since pulling its troops out in 2011. U.S. warplanes bombed Sunni insurgents from the Islamic State, who have marched through northern and western Iraq since June.

Washington says it is taking limited action to protect the Kurdish autonomous region and prevent what Obama called a potential "genocide" of religious minorities targeted by the militants.

The militants made new gains against Kurdish forces despite three days of U.S. airstrikes, while Baghdad, long braced for the Sunni fighters to attack, was now tensing for possible clashes between forces loyal to Prime Minister Nuri al-Maliki and those of his rivals after Iraq's president named a new prime minister on Monday.

Obama says a more inclusive government in Baghdad is a precondition for more aggressive U.S. military support against the Islamic State. He has rejected calls in some quarters for a return of U.S. ground troops, apart from several hundred military advisers sent in June.

The Islamic State, which sees Shi'ites as heretics who deserve to be killed, has ruthlessly moved through one town after another, using tanks and heavy weapons it seized from soldiers who have fled in their thousands.

On Monday, police said the militants had seized the town of Jalawla, 70 miles northeast of Baghdad, after driving out the Kurds' Peshmerga forces.

Washington and its European allies are considering requests for more direct military aid from the Kurds, who have themselves differed with Maliki over the division of oil resources and who took advantage of the Islamists' advance to expand their territory.