The Foreign Secretary has condemned a Syrian mortar attack on a Turkish village that left five people dead including a mother and three of her children.
The Foreign Office said Mr Hague had spoken to Turkish Foreign Minister Ahmet Davutoglu to express his sympathies following the killings in the town of Akcakale on the Turkish-Syria border on Wednesday.
Ankara responded to the shelling by launching its own reprisal artillery attacks on Syrian targets.
Speaking after the violence, the Foreign Secretary said: "I condemn the violence from the Syrian regime which has led to the deaths of a number of Turkish citizens, including a mother and her children. This was an outrageous act. We demand that the Syrian regime avoid any repetition of the incident on the border with Turkey."
He continued: "The events are a stark reminder of the deteriorating situation in Syria, the dangers it presents to the wider region and the need for an urgent resolution of the United Nations Security Council."
Expressing his "deepest sympathies" for the families and communities of those died and at least 10 others who were injured, Mr Hague added: "Turkey is one of our closest allies and key partners in the region, and we will continue to work closely together on the Syria crisis and remain in close touch over the coming days".
US Secretary of State Hillary Clinton said the US was "outraged that the Syrians have been shooting across the border".
Nato's National Atlantic Council, which is composed of the national ambassadors, held an emergency meeting in Brussels on Wednesday night following a request from Ankara. The meeting ended with a statement strongly condemning the attack. It said: "The alliance continues to stand by Turkey and demands the immediate cessation of such aggressive acts against an ally." It also urged the Syrian regime to "put an end to flagrant violations of international law".
On a visit to Turkey, Deputy Prime Minister Nick Clegg said he was "really shocked and saddened" by the deaths of Turkish citizens in the attack, adding: "It's just yet another example of the cruelty and barbarism of the violence which is raging within Syria, yet another example of the totally illegitimate use of force by the Assad regime and yet another reason why we must redouble our efforts working together - the United Kingdom and Turkey - with other members of the international community to bring this cruel style of violence to an end."