The United Nations secretary-general welcomed airstrikes against militants in Syria which he said pose "an immediate threat to international peace and security."

In opening remarks at a climate summit press conference Tuesday, Ban urged world leaders gathered in New York "to come together decisively" in support of efforts to confront extremist groups.

The U.S. and five Arab nations carried out airstrikes against targets of the Islamic State group in Syria on Monday and early Tuesday for the first time, using land- and sea-based U.S. aircraft as well as Tomahawk cruise missiles launched from two Navy ships in the region.

"I welcome the international solidarity to confront this challenge," Ban said.

He cautioned, however, that parties involved in the airstrikes must abide by international humanitarian law and take all precautions to avoid and minimize civilian casualties.

Ban said that although the strikes were not carried out at the direct request of the Syrian government, authorities were informed beforehand. Damascus has said the U.S. informed it that the strikes were coming.

"I also note that the strikes took place in areas no longer under the effective control of that government," Ban said.

"I think it is undeniable — and the subject of broad international consensus — that these extremist groups pose an immediate threat to international peace and security," he added.