Russian President Vladimir Putin is expected to be pressed by EU officials to take a stronger line on the crisis in Syria during a summit in St Petersburg.
EU member states want Russia to put pressure on its ally to withdraw heavy weapons from cities and comply fully with UN envoy Kofi Annan’s peace plan.
Russia and China are also resisting US and European calls to condemn President Bashar al-Assad and seek his removal.
On Sunday, Bashar al-Assad denied his forces had any role in the Houla massacre.
In a televised address, President Bashar al-Assad told parliament the killing of more than 108 people in their homes, including 49 children, was an “ugly crime” that even “monsters” would not carry out.
Witnesses have blamed pro-government militiamen for the massacre, which has triggered international condemnation and led to several countries expelling Syrian diplomats in protest.
Bashar al-Assad said the only way to resolve the crisis was through political dialogue, and that “foreign meddling” was to blame for Syria’s divisions.
European Council President Herman Van Rompuy, European Commission President Jose Manuel Barroso and EU foreign policy chief Catherine Ashton are among those attending Monday’s summit.
On Sunday, Vladimir Putin invited the EU leaders for dinner ahead of the talks at a lavish estate on the outskirts of the city.
European diplomats regard the meeting as a chance to renew ties with Vladimir Putin since his return to the presidency earlier this month.
The leaders are also expected to discuss trade and Iran’s controversial nuclear programme. Russia will also be looking to speed up moves towards visa free travel in Europe.
“We need to make sure that Russia is using fully its leverage in convincing the [Assad] regime to implement [the peace plan],” an EU official quoted by the Reuters news agency said.
“The Russian side has certainly not been very helpful in finding solutions in terms of a political way out.”
Moscow insists it is not protecting Bashar al-Assad but says his removal cannot be a precondition for political dialogue.
Baroness Catherine Ashton, who met Russian Foreign Minister Sergei Lavrov before Sunday’s dinner, said in a statement: “Russia’s role is crucial for the success of Annan’s plan.”
She said the EU wanted to “work closely with Russia to find a way to end the violence”.
The statement added that Baroness Catherine Ashton had spoken to Kofi Annan by telephone on Sunday and they had agreed that the crisis was at “a critical point”.
Analysts say pressure is growing on Moscow to concede that the initiative is stalled and to promote a compromise in which President Bashar al-Assad stands down to allow a transition of power.
US Secretary of State Hillary Clinton said on Sunday that she had “made it very clear” to Sergei Lavrov in a telephone conversation that the focus was shifting to a political transition.
“Assad’s departure does not have to be a precondition, but it should be an outcome so the people of Syria have a chance to express themselves,” she said during a visit to Stockholm.
Although the summit is not expected to produce any major breakthrough in relations between Russia and the EU, it is still important.
EU leaders will be able to reacquaint themselves with Vladimir Putin and it is also a chance to gauge what kind of relationship Moscow and Brussels are likely to have during his six-year presidency.