Theresa May has accused Russia of committing "sickening atrocities" during a bombing campaign that has pummelled the Syrian city of Aleppo.
The Prime Minister made her comments as she arrived for her first European summit, demanding EU leaders take a firm line with Russian President Vladimir Putin.
It also comes as a Russian naval taskforce, including the aircraft carrier Admiral Kuznetsov, was being shadowed by the Royal Navy while it headed towards the eastern Mediterranean and on to Syria.
Ms May demanded a "robust and united European stance in the face of Russian aggression".
She added: "It is vital that we work together to continue to put pressure on Russia to stop these appalling atrocities, these sickening atrocities, in Syria."
Syrian forces, backed by Russian air power, have agreed a temporary humanitarian truce in Aleppo. Mr Putin has held out the prospect of extending it following a meeting with French president Francois Hollande and German chancellor Angela Merkel.
But Nato has said the prospect of Russia's only aircraft carrier heading to the region does not "inspire confidence" that Moscow is seeking a political solution to the Syrian crisis.
An official for the alliance said: "Russia has the right to operate in international waters. There are plans in place for Nato navies to monitor the Russian ships as they head for the Mediterranean."
Type 45 destroyer HMS Duncan sailed from Portsmouth on Tuesday to "man-mark" the Kuznetsov group, as it travelled south through the North Sea and English Channel. The Type 23 frigate HMS Richmond escorted the group from the Norwegian Sea as it steamed south.
A Ministry of Defence spokesman said: "When these ships near our waters we will man-mark them every step of the way. We will be watching as part of our steadfast commitment to keep Britain safe."
Russian defence minister Sergei Shoigu said in September that the Admiral Kuznetsov-led Northern Fleet would be joining a taskforce in the Mediterranean.
According to the Russian news agency Tass, he told a defence board meeting that the plan was to bolster the Mediterranean fleet's "combat capabilities".
A statement from the fleet to the agency on October 15 said the group also consisted of the Pyotr Velikiy battlecruiser, the Severomorsk anti-submarine ship, the Vice-Admiral Kulakov destroyer and other support vessels.
