Islamic State fighters

VIDEO: Russia Attacks May Have Hit Non-IS Areas, says Carter

Carter made the comment at a Pentagon news conference, where he offered few details about the strikes but said they point out a contradiction in Russia's approach. He said the Russians should not be supporting Syrian President Bashar Assad's government.

Carter also expressed disappointment that the Russians did not use formal channels to provide the U.S. with advance notice of its airstrikes.

US warns Russia against striking non-IS groups in Syria

Kerry told the United Nations Security Council that the U.S. would not object to Russians hitting Islamic State or al-Qaida targets but airstrikes just to strengthen the hand of Syrian President Bashar Assad would be worrisome.

"It does appear they were in in areas where there probably were not ISIL forces," Carter said of the Russian airstrikes, using an alternative acronym for Islamic State. He would not go into more detail.

UK drone strike kills 3 Islamic State fighters in Syria

He told Parliament that the attack was legally justified because the militants were plotting lethal attacks against Britain and the fighters could not be eliminated any other way.

"There was a terrorist directing murder on our streets and no other means to stop them," Cameron said, adding that the decision to launch the attack hadn't been taken lightly.

Activists: Hundreds of Syrian Christians flee Islamic State

A U.S.-led coalition has conducted nearly 6,000 airstrikes against the Islamic State group, expanding its operations to target the extremists in Syria as well. But a year later, the Islamic State group remains able to launch attacks across its self-declared "caliphate" in both countries, despite some gains by Kurdish fighters and allied Iraqi forces.

Meanwhile, searches continued in Egypt for a missing Croatian hostage that an Islamic State affiliate had threatened to kill.