EU officials are expected to press Russian President Vladimir Putin to take a stronger line on Syria during a summit in St Petersburg.
EU nations want Russia to put pressure on its ally to withdraw heavy weapons from cities and comply fully with envoy Kofi Annan’s peace plan.
Russia and China are also resisting US and EU 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.
He told parliament 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 hosted 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.
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.
Russia’s role ‘crucial’
“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 Reuters 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 Mr Assad but says his removal cannot be a precondition for political dialogue.
Baroness 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”.
President Assad blames outside forces for causing divisions in his country
The statement added that Baroness Ashton had spoken to Kofi Annan by phone 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 peace plan is stalled and to promote a compromise in which President 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 Mr Lavrov in a phone call 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.
The BBC’s Steve Rosenberg in Moscow says that 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 reacquainted 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, our correspondent adds.
In a televised address, President Assad told parliament that the killings in Houla were an “ugly crime” that even “monsters” would not carry out.
More than 100 people, many of them children, died in the attack overnight between 25 and 26 May, most knifed or shot at close range.
The massacre has triggered international condemnation and led to several countries expelling Syrian diplomats in protest.
Mr Assad reiterated that the only way to resolve the crisis was through political dialogue.
Mr Annan, envoy for the UN and Arab League, has expressed frustration that Mr Assad was not turning his words into actions.
The UN currently has a team of observers in Syria trying to monitor a ceasefire, but violence has continued across the country.