A Syrian opposition leader's pleading for the EU and the international community to do more, amid a deadly bombardment of a rebel enclave outside Damascus.

During a visit to Brussels, Nasr al-Hariri said what's happening in eastern Ghouta amounts to a "war crime."

The delegation head at stalled UN peace talks has met EU foreign policy chief Federica Mogherini.

"Until we see a concerted effort by our allies to systematically leverage the regime and its backers, the Syrian conflict will continue, more refugees will flee their homes, the threat of extremism will remain global," said al-Hariri, President of the Syria Negotiation Commission.

The EU says there's no military solution to the conflict - and that all parties should seriously engage in the UN-led political process.

Al-Hariri added: "There really needs to be a sincere international determination that forces the Syrian regime to negotiate or for its allies to come to negotiate in its place.

"In any case the Syrian regime has no decision to make, the real decision is in the hands of Iran and Russia. If Russia does not come to negotiations, we can not count on the Syrian regime to come and negotiate alone."

The EU will be hoping to keep the Syria crisis high on the international agenda, as it hosts a second Brussels conference on the country's future in April. This, as a resolution still appears out of reach.