Theresa May has told European leaders they must act to stop Russia's appalling behaviour in Syria amid fears some countries are dragging their feet.
The Prime Minister has urged the EU to send a "robust united message" to Moscow over its bombing campaign supporting Syrian dictator Bashar Assad.
At a Brussels summit, Mrs May told counterparts the current assault on Aleppo was "particularly horrific" and Russia's actions had "undermined the West's efforts" to provide a political settlement, according to No 10 sources.
Although the EU imposed sanctions on Russia following the conflict in eastern Ukraine, it has failed to act over Moscow's military intervention in Syria.
Mrs May called on leaders to look for ways to "keep up the pressure by remaining open minded, by being willing to consider things".
Travel bans on military leaders behind the atrocities and sanctions are among the powers the EU could use on Russia.
The calls for action come as Mrs May faced European leaders together for the first time.
French president Francois Hollande warned that the UK would face "hard" negotiations if it wanted to make a clean break with the bloc and European Parliament president Martin Schulz urged European leaders to “stand firm” in its negotiations with Britain.
Leaders will discuss trade rules when the European Council continues on Friday, including anti-dumping measures.
Mrs May will then hold talks with European Commission president Jean-Claude Juncker, which No 10 sources said would be an “opportunity to establish a relationship” and talk about “how we see the process ahead”.