Theresa May has warned Russia it may face further sanctions if international humanitarian law is breached in Syria.
The Prime Minister's comments came after European leaders met outgoing US president Barack Obama for talks in Berlin amid concerns about his successor Donald Trump's attitude to Moscow and commitment to the Nato alliance.
Mrs May said the leaders of the US, UK, Germany, France, Italy and Spain were united in condemnation of the atrocities in Aleppo, where Bashar Assad's Syrian forces are being aided by Moscow in attacks on rebel-held areas.
The summit in Berlin marked Mr Obama's final trip to Europe as president and was aimed at discussing some of the many global challenges that will remain when Mr Trump takes office in January.
Appearing alongside Angela Merkel at the Chancellery in the German capital, Mrs May said: "On Syria, looking at Aleppo, we are united in our condemnation of the atrocities that are taking place there.
"We agree on the need to keep up the pressure on Russia, including the possibility of sanctions on those who breach international humanitarian law."
The White House said Mr Obama urged the European leaders at the meeting - Mrs May, Mrs Merkel, French president Francois Hollande, Italian prime minister Matteo Renzi and Spanish prime minister Mariano Rajoy - to work with Mr Trump when he takes office.
There is nervousness in Europe about Mr Trump's criticism of Nato and praise for Russian leader Vladimir Putin.
A White House statement said: "The leaders agreed on the necessity of working collectively to move the transatlantic agenda forward, particularly on bringing stabilisation to the Middle East and North Africa, as well as securing diplomatic resolution to the conflicts in Syria and eastern Ukraine.
"The leaders also affirmed the importance of continued cooperation through multilateral institutions, including Nato."
On his farewell visit to the continent, Mr Obama "urged his European counterparts to continue seeking solutions to common challenges with the incoming US administration on the basis of the core values that define the United States and Europe as open democracies", the White House said.
The Prime Minister said she would update Mrs Merkel on the Brexit process - even though the German chancellor said it was not on the agenda.
Mrs Merkel said : "We will not discuss any further today the British wish to leave the (European) Union.
"That will not be on the agenda because the Prime Minister has already assured us at the very latest that Britain will invoke Article 50 by the end of March."
She said the Germans would wait until the motion had been tabled for further talks.
But Mrs May said: "I will be able to update Chancellor Merkel on where we are on our Brexit preparations.
"Our work is on track, we do stand ready to trigger Article 50 by the end of March 2017, that's next year.
"I want to see this as a smooth process, an orderly process, working towards a solution that is in the interests of both the UK but also in the interests of our European partners too."
The meeting follows a fresh warning from Berlin about the difficulties Mrs May will face as she attempts to secure a Brexit deal.
Germany's finance minister Wolfgang Schauble said the UK faces paying into the European Union for more than a decade after it quits the bloc.
He added that there will be no special deal to curb freedom of movement if the UK wants to remain part of the common market.
Responding to Mr Schauble's remarks, a Downing Street spokesman said: "We have always set out that as long as we remain members of the EU, we will fulfil the commitments and obligations to the EU. What happens once we have left the EU is a matter for the negotiation process."
During their bilateral talks, which lasted more than an hour, Mrs May and Mrs Merkel agreed on the need to work with Mr Trump.
A Number 10 spokesman said: "Following on from the morning meeting, they agreed on the enduring importance of the transatlantic relationship and looked forward to their future cooperation with President-elect Donald Trump on the range of global challenges."
They also agreed the need to "strengthen the links between the people of the UK and Germany, both while we remain in the EU and once the UK has left".