The White House refused to say Monday whether U.S. President Barack Obama will seek explicit congressional authority for military strikes on ISIS militants inside Syria. 

Earlier Monday, Syria's foreign minister warned the U.S. not to conduct airstrikes inside Syria without the consent of the Syrian regime, even as the Islamic State in Iraq and Syria seized control of a major air base.

Walid al-Moallem says such an act "by anyone," without the approval from President Bashar al-Assad's government, would be a violation of Syrian sovereignty and would be considered an aggression.

"Syria is ready to cooperate and coordinate on the regional and international level in the war on terror," al-Moallem said. "But any effort to combat terrorism should come in coordination with the Syrian government."

'I think ISIS has been so brutal, and has wrapped itself in a radical religious legitimacy.'- Martin Dempsey, U.S. General

U.S. Gen. Martin Dempsey said Sunday that once he determines the Islamic State (ISIS) militants in Iraq have become a direct threat to the U.S. homeland, he will recommend the U.S. military move directly against the group in Syria.

But the chairman of the Joint Chiefs of Staff said that right now, he still believes the insurgent group is still more a regional threat and is not plotting or planning attacks against either the U.S. or Europe.

Dempsey, speaking on a military plane en route to Afghanistan, did not rule out strikes for any other critical reasons, but listed a homeland threat as one of the key triggers for any military action in Syria.

Dempsey also told reporters travelling with him that he believes that key allies in the region — including Jordan, Turkey and Saudi Arabia — will join the U.S. in quashing the Islamic State group.

"I think ISIS has been so brutal, and has wrapped itself in a radical religious legitimacy that clearly threatens everybody I just mentioned, that I think they will be willing partners," said Dempsey, expressing optimism for the first time that the Arab nations would join in the conflict.

Syria had been quiet on ISIS threat

Al-Moallem's remarks at a news conference in Damascus marked the first public comments by a senior Assad official on the threat posed by the Islamic State group, which has captured large swaths of Iraqi and Syrian territory.

Syria crisis

A Tabqa city resident waves an Islamist flag to celebrate after ISIS militants reportedly seized control of the Tabqa air base, in nearby Raqqa city on Monday. (Reuters)

His comments follow the capture by ISIS of a major military air base in northeastern Syria, eliminating the last government-held outpost in a province otherwise dominated by the militants.

Unverified video showed ISIS fighters clashing with Syrian forces at Tabqa airbase. An Al Jazeera report said the militants later captured the base in a battle that claimed more than 500 lives.

ISIS has established a self-declared caliphate in areas straddling Iraq and Syria's shared borders. The United States began airstrikes against the group in northern Iraq earlier this month, and is now considering similar strikes against the militants in Syria.

The Obama administration remains wary, however, of getting dragged into the bloody and complex Syrian civil war that the United Nations says has killed more than 190,000 people.

Al-Moallem said his government is ready to cooperate with any side, including the U.S., or join any regional or international alliance against the Islamic State group. But he said any military action inside Syria should be coordinated with the government, "which represents Syrian sovereignty."

"Any strike which is not coordinated with the government will be considered as aggression," he said.

Syria condemns journalist's killing

American Freed Syria

Peter Theo Curtis, a U.S. freelance reporter, was freed after being held by a Syrian rebel group for almost two years. (Associated Press)

​Al-Moallem also denounced "in the strongest terms possible" the killing last week of U.S. journalist James Foley by Islamic State militants.

"We condemn the killing of Foley, but may I ask has the West ever condemned the massacres by the Islamic State and Nusra against our armed forces or citizens?" he asked.

He also welcomed the release Sunday of U.S. freelance reporter Peter Theo Curtis, who had been held hostage for nearly two years by the Nusra Front. He slammed the oil-rich nation of Qatar, which said Sunday that it has "exerted relentless efforts" to win his freedom.

"Don't these efforts constitute proof that they (Qataris) are linked to the Nusra terrorist organization" he asked.

Qatar is a leading supporter of the Syrian rebels fighting to oust Assad and has been involved in mediating past hostage releases. The country's foreign minister said Saturday that Qatar "does not support extremist groups," including the Islamic State group, in any way.

UN accuses ISIS of ethnic, religious cleansing

Also Monday, United Nations human rights chief Navi Pillay condemned the "appalling, widespread" crimes being committed by ISIS.

The persecution of entire communities and systematic violations by the al-Qaida offshoot, documented by UN human rights investigators, would amount to crimes against humanity and war crimes under international law, Pillay said in a statement.

"Grave, horrific human rights violations are being committed daily by (ISIS) and associated armed groups," she said, citing targeted killings, forced conversions, abductions, slavery, sex crimes, forced recruitment and destruction of places of worship.

"They are systematically targeting men, women and children based on their ethnic, religious or sectarian affiliation and are ruthlessly carrying out widespread ethnic and religious cleansing in the areas under their control."

Christians, Yazidis and Turkmen were among the minorities targeted by the Sunni militant group, which has forced people to convert to their strict form of Sharia law, she said.

Some 1.2 million people have fled fighting and ISIS’s advance in Iraq this year, the UN’s refugee agency says.

With files from CBC News and Reuters