BAGHDAD—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 in Baghdad led by Nouri al-Maliki.

The latest military 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 Qaeda breakaway group began 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 (ISIL) takes the group closer to its dream of carving out an Islamic state straddling both Syria and Iraq. Moreover, controlling the borders with Syria will help it supply fellow fighters there with weaponry looted from Iraqi warehouses, significantly reinforcing 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 Qaeda attracted extremists in Taliban-ruled Afghanistan.

U.S. 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 Middle Eastern nations, most of them led by Sunni governments, to curb the Sunni militant group’s growth. Officials in the U.S. 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. However, he has been uncharacteristically silent since Obama and Iraq’s top Shiite cleric both urged the prime minister 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, when it will elect 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 and their inability to defend the rapidly shrinking territory they hold. In the north, troops fled in the face of the 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.

“We have a very, very serious crisis to deal with,” acknowledged a senior government official close to al-Maliki’s inner circle. “Up until now, we don’t have a plan to retake any territory we lost. We are working on one still.”

A top Iraqi military intelligence official was equally blunt, saying the battlefield setbacks in Iraq’s restive 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 defences,” 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 or turn things around quickly enough to prevent the militants from digging in and improving their defences. 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 co-operate in addressing the shared threat posed by the Islamic extremists.

“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.”

“The U.S. aims to bring its own blind followers to power,” said Khamenei, whose Shiite, none-Arab nation has close ties with al-Maliki’s government and effectively plays the role of guarantor for Iraq’s Shiite political domination.

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

For now, however, the militants are on a seemingly unstoppable offensive.

On Sunday, their military advances took the conflict in Iraq to the doorstep of Jordan, a key U.S. ally that also borders embattled Syria to its north.

Sunday’s capture of crossings bordering Jordan and Syria follows 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 neighbour. It is a key artery for passengers and goods and has been infrequently used in recent months because of deteriorating security.

The capture of Rawah on the Euphrates River and the nearby town of Anah appeared to be part of a 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 military has dispatched reinforcements to the dam’s site to protect it.

In other violence, Sunni militants in control of a small northern town handed over the decomposing bodies of 15 Shiites to authorities in the northern city of Kirkuk, according to the city’s deputy police chief, Maj. Gen. Torhan Abdul-Rahman Youssef.

Residents of the town of Besher said the Shiites were hung from street lights and a water tank for days. The circumstances of their deaths were not immediately known and the residents requested anonymity for fear of reprisals.