EU officials are expected to press Russian President Vladimir Putin 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, Mr Assad denied his forces had any role in the Houla massacre.
In a televised address, President 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.
Mr 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, Mr 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 Mr 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.
But correspondents say Syria is likely to dominate the agenda.
