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Sunni militants have blitzed through the vast desert of western Iraq, capturing four towns and three border crossings and deepening the predicament of the Shiite-led government of Prime Minister Nouri al-Maliki.

Their latest victories — including two border posts captured Sunday, one along the frontier with Jordan and the other with Syria — considerably expanded territory under the militants' control just two weeks after the al-Qaida breakaway group started swallowing up chunks of northern Iraq, heightening pressure on al-Maliki to step aside.

The lightening offensive by the Islamic State of Iraq and the Levant takes the group closer to its dream of carving out an Islamic state straddling both Syria and Iraq. Controlling the borders with Syria will help it supply fellow fighters there with weaponry looted from Iraqi warehouses, boosting its ability to battle beleaguered Syrian government forces.

If the Sunni insurgents succeed in their quest to secure an enclave, they could further unsettle the already volatile Middle East and serve as a magnet for Jihadists from around the world — much like al-Qaida attracted extremists in Taliban-ruled Afghanistan.

President Barack Obama, in an interview with CBS' "Face the Nation" aired Sunday, warned that the Islamic State could grow in power and destabilize the region. Washington, he said, must remain "vigilant" but would not "play whack-a-mole and send U.S. troops ... wherever these organizations pop up."

U.S. Secretary of State John Kerry, in the Jordanian capital on Sunday, also weighed in. The Islamic State, he warned, is a "threat not only to Iraq, but to the entire region."

The U.S. is looking for ways to work with Mideast nations, most of them led by Sunni governments, to curb the Sunni militant group's growth. Officials in the United States and the Middle East have suggested privately that al-Maliki must leave office before Iraq's Sunnis will believe that their complaints of marginalization by the Shiite-led government in Baghdad will be addressed.

Al-Maliki, in office since 2006, has shown no sign he is willing to step down. But he has been uncharacteristically silent since Obama and Iraq's top Shiite cleric both urged him last week to form an inclusive government that promotes the interests of all of Iraq's ethnic and religious groups.

Iraq's newly elected parliament must meet by June 30 to choose a speaker and a new president, who, in turn, will ask the leader who enjoys the support of a simple majority in the 328-seat chamber to form a new government. Al-Maliki's State of the Law won 92 seats, more than any other group but not enough to form a government.

The militants' stunning battlefield successes in the north and the west of Iraq have laid bare the inadequacies of the country's U.S.-trained forces. In the north, troops fled in the face of advancing militants, abandoning their weapons, vehicles and other equipment. In some cases in the west, they pulled out either when the militants approached or when they heard of other towns falling.

The chief military spokesman, Lt. Gen. Qassim al-Moussawi, spoke on Sunday of tactical withdrawals to regroup and prepare to retake what has been lost to the militants.

"Up until now, we don't have a plan to retake any territory we lost. We are working on one still," acknowledged a senior government official close to al-Maliki's inner circle.

A top Iraqi military intelligence official was equally blunt, saying the battlefield setbacks in the western Anbar province and the north have given the militants much more freedom of movement and their firepower has dramatically increased.

"Their objective is Baghdad, where we are working frantically to bolster our defenses," said the official. "I will be honest with you, even that is not up to the level of what is needed. Morale is low."

Both officials spoke on condition of anonymity to discuss the sensitive subject.

It is not clear whether Obama's deployment of up to 300 military advisers to retrain Iraqi troops could make a difference. Obama has also left the door open for airstrikes.

Iran's supreme leader, Ayatollah Ali Khamenei, said he was opposed to any U.S. involvement in the Iraqi crisis, accusing Washington of fomenting the unrest. His comments appeared to quash recent speculation that the two rivals might cooperate in addressing the shared threat posed by the Islamic extremists.

The U.S. has long accused the Shiite, non-Arab Iran of meddling in Iraq, including organizing and backing Shiite militias following the 2003 invasion.

"We strongly oppose the intervention of the U.S. and others in the domestic affairs of Iraq," Khamenei, who has the final say over Iran's state policy, was quoted as saying by the IRNA state news agency, in his first reaction to the crisis. "The main dispute in Iraq is between those who want Iraq to join the U.S. camp and those who seek an independent Iraq."

Sunday's capture by the militants of crossings bordering Jordan and Syria followed the fall on Friday and Saturday of the towns of Qaim, Rawah, Anah and Rutba, all of which are in Sunni-dominated Anbar province, where the militants have since January controlled the city of Fallujah and parts of the provincial capital, Ramadi.

Rutba is on the main highway from Baghdad to the two border crossings and the capture of the crossing into Jordan has effectively cut the Iraqi capital's main land route to its neighbor.

The militants now appear poised to march toward a key dam in the city of Haditha, the destruction of which would damage the country's electrical grid and cause major flooding. The Iraqi military has dispatched reinforcements to the dam's site to protect it.