US Secretary of State John Kerry was among ministers and officials from 17 countries who, along with the UN, the EU and the Arab league, called for an immediate ceasefire across Libya.
They pledged support for efforts to end the chaos that has engulfed the North African state since the 2011 overthrow of Moamer Kadhafi, and to reduce the risk of Islamic State (IS) expanding its presence.
"It is dangerous for the viability of Libya, dangerous for Libyans and, because of Daesh (IS) migrating there, dangerous for everyone.
"We refuse to stand by and watch a vacuum filled by terrorists."
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Kerry dismissed criticism that the proposed Government of National Accord was being imposed from outside in the absence of consensus on the ground, and amid a security situation which makes it uncertain that the new administration will be able to base itself in the capital Tripoli.
The ministers were joined in the afternoon by the Libyan officials from factions in the country's rival parliaments, an internationally-recognised one based in Tobruk and an Islamist-backed assembly in Tripoli.
Some representatives of both assemblies have tentatively agreed to sign the UN plan in Morocco on Wednesday but it remains unclear how much opposition it will encounter.
Kerry insisted that majorities of both parliaments supported the peace plan and said only fringe players were obstructing progress.
The figures who had come to Rome "have decided they are not going to let Libya be the prisoner of one or two people or small groups," he said.
The accelerated diplomacy reflects fears in Western capitals that instability in Libya will allow IS-allied groups to expand beyond a small section of coastline they currently control around the city of Sirte.