Russia Says U.S. Strikes in Syria Would Violate International Law

"In the absence of an appropriate decision of the U.N. Security Council, such a step would become an act of aggression, a crude violation of the norms of international law," Alexander Lukashevich, a spokesman for the Russian Foreign Ministry, told a regular briefing.

U.S. President Barack Obama on Wednesday authorized the start of U.S. airstrikes in Syria and expanded a monthlong bombing campaign in Iraq to "degrade and ultimately destroy" Islamic militants who recently beheaded two Americans.

Moscow, which has long supported the government of Syrian President Bashar al-Assad, suspects Washington would use such strikes to attack government forces, Mr. Lukashevich said. "There is reason to suspect that Syrian government forces could also come under fire with serious consequences for further escalation of tensions," he said.

Governments elsewhere responded to Mr. Obama's announcement by offering their support.

"We need to deal with ISIL [Islamic State] both in terms of Iraq and in terms of Syria, so we are very supportive of that approach," said a spokesman for U.K. Prime Minister David Cameron.

Australian Prime Minister Tony Abbott also offered his backing, saying Islamic State posed a threat not only internationally, but at home, with at least 60 Australians having already joined insurgent ranks in the Middle East.

"This is at least as much a domestic security issue for us as it is an international security issue," Mr. Abbott told reporters in the southern state of Tasmania. "This is not a conflict which is remote to us."

Japan also said it would continue offer to humanitarian assistance to the American-led campaign. On Thursday morning, a Foreign Ministry spokeswoman said the Japanese government has been critical of every form of terrorism, including attacks by Islamic State insurgents in Syria and Iraq.

"The Japanese government will continue to closely coordinate with the United States and other countries, and will offer its support and cooperation," press secretary Takako Ito said.

The initial reaction in the Middle East to Mr. Obama's announcement was muted. Arab states of the Gulf Cooperation Council have previously expressed skepticism of U.S. rhetoric for further intervention against Islamic State, without knowing the full details of U.S. participation or the role expected of their own militaries.

"We are now seeing clearly that there is a determined president who wants to take a lead. This is a chance for America to make a comeback," said Abdulkhaleq Abdulla, professor of political science at Emirates University in Al Ain. "This is a blessing in disguise."

U.S. forces have launched more than 150 airstrikes against Islamic State fighters in Iraq since early August, a period in which two American journalists have been beheaded by the militant group. The U.S. will also now begin airstrikes against the militants in Syria and Mr. Obama has pledged training and support for moderate Syrian fighters battling the extremist group.

Yousef Al Otaiba, the U.A.E.'s ambassador to the U.S., this week labeled Islamic extremism "the most destabilizing and dangerous global force since fascism" in an opinion piece published in The Wall Street Journal.

Saudi Arabia and the U.A.E. advocate a collective international response to all Middle East groups designated as terrorist organizations, including Al Nusra Front in Syria, Ansar al Shariah in Libya and Tunisia, Ansar Bayt al Maqdis in Egypt and al Qaeda in the Yemen and North Africa.

The U.S., meanwhile, views support in its campaign from Saudi Arabia, the U.A.E., Qatar, Turkey and Jordan as crucial in cutting off the flow of money and foreign fighters to Islamic State. But the governments of Saudi Arabia and the U.A.E. have been frustrated in recent years with what they view as a withdrawal of U.S. influence, power and support in the region.

—Nicholas Winning in London, Rob Taylor in Canberra and Alexander Martin in Tokya contributed to this article.