European Union foreign ministers have called for a calm response after Turkey accused Syria of shooting down one of its military reconnaissance planes in international airspace.
The call came as the ministers, meeting in Luxembourg on Monday, announced further sanctions against Syria.
"We're very concerned about what's happened and very concerned for the family of the two pilots who are missing," EU foreign policy chief Catherine Ashton said on Monday.
"We will be obviously looking to Turkey to be restrained in its response."
The ministers were meeting a day before a NATO meeting requested by Turkey to discuss how to react to Friday's incident, which Turkey says occurred without warning.
Turkey is a member of NATO, but is only a candidate for membership of the European Union.
In-depth coverage of escalating violence across Syria
The ministers added one person and six companies and other entities to a sanctions list following Monday's meeting, an EU official said.
So far, the EU has imposed asset freezes and visa bans on more than 100 people responsible for, or associated with, the violent repression against Syria's civilian population, and an asset freeze on 43 entities.
William Hague, the British foreign secretary, called for increased pressure, but said the jet incident did not fundamentally alter the situation in Syria, where Assad has been suppressing a 16-month revolt against his rule.
"I don't think it illustrates a different phase," Hague said. "It's very important that we increase the pressure with additional sanctions.
"Other countries will be very active in arguing for a new resolution from the [UN] Security Council."
Russia and China, both permanent members of the UN Security Council with veto power, have blocked efforts by Western powers to condemn Assad or call for his removal.
Laurent Fabius, the French foreign minister, said on Monday that Syria's downing of the jet was "unacceptable".
"This plane was not carrying arms and was on a routine flight ... there was no prior warning," Fabius said. "This is completely unacceptable."
Selcuk Unal, spokesperson for the Turkish foreign ministry told Al Jazeera that the shooting down of the Turkish jet was "a hostile act".
Turkey's cabinet was due to meet on Monday to discuss Friday's incident.
"Turkey has moved into action" the country's Milliyet and Vatan newspapers declared in headlines under the NATO flag.
Sabah newspaper columnist Mehmet Barlas said some were calling Friday's attack a Syrian declaration of war.
"We alreadyknow there is an 'undeclared war' being carried out between Turkey and Syria," he said, citing the presence of the Syrian opposition and Syrian refugees sheltering on Turkish soil.
'Out of the question'
Recep Tayyip Erdogan, Turkey's prime minister, has so far eschewed bellicose rhetoric, and has seemed to back away from any suggestion of an armed response.
In calling Tuesday's meeting he invoked a NATO article providing for urgent consultations if a member state considers its security interests threatened.
"Military intervention in Syria is out of the question," said Uri Rosenthal, the Dutch foreign minister. "It is not a matter of consideration for the Dutch government. That is also at stake in the ... context of NATO."
The UN has said more than 10,000 people have been killed by government forces, while Syria has said at least 2,600 members of the military and security forces have died at the hands of what it calls foreign-backed "terrorists".
The recent intensification of fighting has raised fears in Turkey of a further flood of refugees and a slide into ethnic and religious warfare that could envelop the region.
Ankara, like the West, is torn between a wish to remove Assad and the fear that any armed intervention could unleash such forces.