NATO has demanded an immediate halt to "aggressive acts" against alliance member Turkey after a mortar strike from Syria killed five Turkish civilians.
Turkey said it had struck targets inside Syria in response to the mortar fire and Britain's UN Ambassador Mark Lyall Grant called the situation "very worrying."
He said the crisis was raised briefly during a meeting of the 15-nation UN Security Council devoted to other issues.
Syria has said it is investigating the source of the shell that hit Turkey and called on its neighbours to respect its sovereignty and stop "terrorists" from crossing into the country.
The shelling "constitutes a cause of greatest concern for, and is strongly condemned by, all allies", NATO ambassadors said in a statement, after they held a rare late-night meeting at Turkey's request to discuss the incident.
"The alliance continues to stand by Turkey and demands the immediate cessation of such aggressive acts against an ally, and urges the Syrian regime to put an end to flagrant violations of international law," the statement said.
UN Secretary-General Ban Ki-moon urged Turkey to keep open all channels of communication with the Syrian government after the attack.
UN spokesman Martin Nesirky told reporters that Mr Ban spoke with Turkey's Foreign Minister Ahmet Davutoglu and "encouraged the minister to keep open all channels of communications with the Syrian authorities with a view to lessening any tension that could build up as a result of the incident."
That statement was issued before Turkey's announcement that it had struck targets inside Syria.
In a second statement issued shortly after Turkey's announcement, Mr Ban urged the Syrian government to respect the territorial integrity of its neighbours and warned that the 18 month-long conflict in Syria was increasingly harming other countries in the region.
At least 40 killed in Syrian explosions
At least 40 people have been killed and 90 injured in a series of explosions in the centre of the northern Syrian city of Aleppo.
State television said four blasts ripped through Aleppo's main Saadallah al-Jabiri Square.
A fifth blast struck a few hundred metres away, on the fringes of the Old City where rebels and forces loyal to President Bashar al-Assad have been fighting.
The State broadcaster showed footage of three dead men disguised as soldiers in army fatigues who it said were shot by security forces before they could detonate explosive-packed belts they were wearing.
One appeared to be holding a trigger device in his hand.
Rebels fighting to overthrow President Bashar al-Assad announced last week a new offensive in Aleppo, Syria's largest city.
Aleppo is commercial hub of 2.5m people, but neither side has appeared to make significant gains so far.
The explosions also came a week after rebels bombed military command buildings in the heart of Damascus and clashed with security forces for several hours.
Aleppo is now split in two with Assad's forces mainly in the west and rebels in the east.
Several large protests in support of the president have been held in Saadallah al-Jabiri square.
Pro-Assad al-Ikhbariya TV showed footage of four dead men, including one dust-covered body being pulled from the rubble of a collapsed building and loaded onto the back of a pickup truck.
Many of the multi-storey buildings on the square had their facades ripped off and there was a deep crater in the road.