By James Rosen and Jonathan S. Landay, McClatchy Washington Bureau

WASHINGTON — Ninety U.S. military advisers arrived in Baghdad on Tuesday to begin assessing Iraqi security forces’ ability to regain control of the embattled country, with the Pentagon acknowledging for the first time that the Iraqi capital could fall to Islamist militants.

The new arrivals joined 40 American troops sent to Baghdad on June 15 to form 130 of the up to 300 U.S. forces that President Barack Obama announced last week he was rushing to Iraq to help counter a two-week offensive by the militants.

Rear Adm. John Kirby, the Pentagon spokesman, said the 130 Americans would take two to three weeks to assess the current security situation and recommend how to dispatch the additional up to 170 military advisers to Iraq.

“It’s a measured, deliberate approach to help us and (Iraqi security forces) get better eyes on the situation and what they’re facing,” Kirby told reporters.

Kirby pushed back at one journalist’s description of such an approach as “rather leisurely,” given how quickly the Sunni insurgents, the Islamic State of Iraq and Syria, or ISIS, have seized a broad swath of cities and territory north of Baghdad.

“Everybody shares a sense of urgency here about what is going on inside Iraq,” Kirby said.

In a separate briefing, a senior U.S. intelligence official said it may be difficult for Iraqi security forces to regain the critical area of central-north Iraq between Tikrit and Mosul. The ISIS fighters number about 3,000 but are being supplemented through alliances with other Sunni groups, including tribesmen at odds with the Shiite-dominated central government of Prime Minister Nouri al-Maliki and former supporters of the late secular dictator Saddam Hussein.

“As long as the support of these Sunni elements holds, (ISIS) looks well-positioned to keep the territory it has captured, absent a major counteroffensive” by Iraqi security forces, said the U.S. intelligence official, who requested anonymity in order to share details of the crisis.

ISIS views the upheaval in Syria and Iraq as a single war between Sunni and Shiite Muslims, intelligence and Pentagon officials said.

“The group appears to be benefiting from a regional strategy that looks at Syria and Iraq as one interchangeable battlefield, allowing it to shift manpower and resources in pursuit of military objectives,” the senior intelligence official said.

ISIS has an additional 7,000 fighters in Syria, with 3,000 to 5,000 of the total 10,000 combined militants in the two countries made up of foreigners, the intelligence official said. Kirby said ISIS fighters are going back and forth between the neighboring countries.

ISIS’ ranks in Iraq are dominated by Iraqis and Syrians, but there are also foreigners who most likely include some of the dozens of Americans who have sought to join the fight in Syria against Syrian President Bashar Assad, the senior U.S. intelligence official said.

“There’s a porous border between Iraq and Syria, which we remain concerned about,” Kirby said. “They flow back and forth across that border to sustain themselves. There’s no question about that.”

Kirby expressed grudging respect for the military prowess of an insurgent group that has marched southward toward Baghdad despite being vastly outnumbered by Iraqi security forces.

“They’re better organized than most other terrorist networks,” Kirby said. “And they are aided by foreign fighters and even just some Sunni sympathizers that aren’t necessarily self-identified members of (ISIS).”

With ISIS and Iraqi security forces battling for control of Iraq’s largest oil refinery, near the insurgent-held city of Tikrit about 110 miles north of Baghdad, Kirby said the capital could come under siege from the militants.

“They continue to press into central and southern Iraq,” Kirby said. “They’re still fighting over this oil refinery, which I think remains contested territory right now. And they’re still a legitimate threat to Baghdad.”

The senior U.S. intelligence official, however, said ISIS fighters would face much stronger resistance in the capital and Iraq’s Shiite-dominated southern region.

“The situation on the ground right now is playing to (ISIS’) strength, but the group faces the real prospect of overstretch if it tries to press deep into Baghdad and beyond,” the intelligence official said.

Kirby said the first task of the U.S. military advisers is to set up a joint operations center in Baghdad to be run with senior American and Iraqi officers. The advisers also will establish a joint operations center in the north of Iraq, Kirby said, but that is a more difficult task because of the ISIS fighters’ gains in the region.

Although Obama announced the dispatch of military advisers Thursday, Kirby confirmed that the Pentagon had held off sending them into Iraq for five days because the Iraqi government had not provided assurances of immunity from criminal prosecution for any of their activities.

Those assurances came Monday, Kirby said, delivered through a formal diplomatic note.

It has been more difficult for the United States to gather intelligence in Iraq since the last U.S. combat brigades left the country at the end of 2011. But the CIA and other agencies have been making use of ISIS posts on Facebook, Twitter and other social media.

“Particularly in this rapidly changing situation, we rely very heavily on what the group is posting itself to give us more insight into the on-the-ground operations,” the senior intelligence official said.

Ahmad al-Rubaye via AFP