"This plane was not carrying arms and was on a routine flight ... there was no prior warning ... this is completely unacceptable," French Foreign Minister Laurent Fabius on Monday was quoted by Agence France-Presse as saying.

US Secretary of State Hillary Clinton called the attack "brazen and unacceptable" and said Washington would cooperate closely with Ankara to promote a transition in Syria.

Meanwhile, European Union foreign ministers on Monday called for a calm response to the dispute and more pressure on Assad.

"We will be obviously looking to Turkey to be restrained in its response," EU High Representative for Foreign Affairs and Security Policy Catherine Ashton was quoted by Reuters as saying, adding that the EU is very concerned about the incident and the families of the two pilots.

Saying the jet incident did not fundamentally alter the situation in Syria, British Foreign Secretary William Hague urged increased pressure on Assad.

"The incident creates a good excuse for the Western allies to conduct military intervention," said Wang Jinglie, an expert on Middle Eastern studies with the Chinese Academy of Social Sciences.

Wang warned that leadership change in Syria might intensify religious conflicts in the region, which will not benefit Western countries.