Airlines in the Eastern Mediterranean were placed on high alert amid the possibility of a US-led airstrike response to a suspected chemical attack in Syria.
It comes as speculation over if a strike would happen continued as Theresa May said Britain would need more evidence before joining a military retaliation over the alleged poison gas attack.
But European Aviation Security Agency warned pilots of possible air-to-ground missile launches as Donald Trump and US allies continued to weigh up retaliatory measures.
It told operators: Please note that EASA has issued Rapid Alert Notification for Eastern Mediterranean/Nicosia FIR (flight information region) area stating that: Due to the possible launch of air strikes into Syria with air-to-ground and/or cruise missiles within the next 72 hours, and the possibility of intermittent disruption of radio navigation equipment, due consideration needs to be taken when planning flight operations in the Eastern Mediterranean/Nicosia FIR area.
The Prime Minister spoke to Mr Trump and Emmanuel Macron on Tuesday, as speculation mounted about what action will be taken.
Mr Trump has said the apparent poison gas attack in Syria will be met forcefully, but did not give a time-frame for any retaliatory action, and he cancelled a foreign trip on Tuesday to manage the crisis.
A spokeswoman said Mrs May held separate calls with the two leaders on Tuesday and that they agreed the reports of a chemical weapons attack in Syria were utterly reprehensible and that they would continue working closely together.
Boris Johnson criticised Russia after it vetoed a US draft resolution at the United Nations which sought to create a new body to determine responsibility for the attack.
Amid speculation over whether Britain will take action against Syria, Tory chairman of the Defence Select Committee Julian Lewis said airstrikes can only be effective when they are in support of ground troops.
Mr Lewis told BBC Two’s Newsnight: “What we’ve got here in Syria is a choice between monsters on the one hand and maniacs on the other, and it is absolutely untrue to say that apart from the Kurdish-led forces, the Salafists and the Jihadists are not in control of the opposition groups.
The United States is looking to the UK and France for support as it finalises its response to the assault on the rebel-held town of Douma.
President Trump and his French counterpart Mr Macron had already agreed to coordinate a strong, joint response after talks by telephone.
She is expected to comment on the situation again on Wednesday during a visit to Birmingham.
The attack in Douma happened late on Saturday amid a resumed offensive by Syrian government forces after the collapse of a truce with the Army of Islam rebel group.
Families were reportedly found suffocated in their homes and shelters, with foam on their mouths.
The attack comes almost exactly a year after a chemical atrocity in the northern Syrian town of Khan Sheikhoun killed dozens of people.
That attack prompted the US to launch several dozen Tomahawk cruise missiles at a Syrian air base.
Russia and Syria have blamed Israel for an attack on a Syrian military airport on Monday that reportedly killed at least 14 people.