Russia already has about 10 ships off Syria, which have fired cruise missiles during Russia's bombardment of what it says are anti-government rebels in Syria.
The temporary truce is part of a plan to allow civilians and fighters to leave, and Russian and Syrian air strikes have been halted since Tuesday.
Prime Minister Theresa May urged European leaders to send a "robust united message" to Moscow over its bombing campaign.
At a Brussels summit on Thursday, Mrs May told her counterparts the current assault on Aleppo was "particularly horrific" and Russia's actions had "undermined the West's efforts" to provide a political settlement, Downing Street sources said.
Although the EU imposed sanctions on Russia following the conflict in eastern Ukraine, it has so far failed to act over Moscow's military intervention in Syria.
