The U.N. Security Council discussed on Wednesday rising tensions between Syria and Turkey after a mortar bomb from Syria landed in Turkey, killing at least five people, Britain's U.N. envoy Mark Lyall Grant said.

"It's very worrying, and it was raised and discussed in the council this afternoon," said Lyall Grant, adding that the council was now awaiting a letter from Turkey on the incident before it considered possible moves.

The 15-member council was already in a meeting to discuss other issues when Turkey announced it had struck targets in Syria in response to the mortar bombing. It is the most serious cross-border escalation of the 18-month uprising in Syria.