At least 55 countries will on Monday demand that the UN Security Council refer the Syria conflict to the International Criminal Court for a war crimes investigation.
The demand will be made in a letter organized by Switzerland, which has spent seven months collecting signatories. The 15-member Security Council is the only body that can refer the case to the ICC.
Swiss UN mission spokesman Adrian Sollberger told AFP the letter would be handed over Monday.
Other diplomatic sources said 55 countries had signed and others could still join even though the initiative has little immediate chance of success.
The Security Council is in a crippling deadlock over the 22-month old Syria conflict. Permanent members Russia and China have vetoed three resolutions which would have threatened sanctions against Syria's President Bashar al-Assad.
As neither are members of The Hague-based court, both countries would almost certainly reject any new resolution proposing war crimes charges. Syria is also not a member.
European countries have provided the majority of the signatories, according to diplomatic sources.
The United States has not signed the letter because it is not an ICC member, but does support the initiative, diplomats said.
"Our proposal is supported throughout Europe and also in most other regions of the world," Switzerland's Foreign Minister Didier Burkhalter told SRF Swiss broadcaster on Friday. "We can get this operation moving now," he added.
"There are horrific war crimes happening in Syria," Burkhalter said. "People must realize that these crimes will not go unpunished."
Russia has been the main opponent of international action on the conflict, which the UN estimates has now left more than 60,000 dead.
The Russian government reaffirmed Sunday that it opposes any move to force Assad from power as part of any deal to get talks started on a political settlement.
Assad's ouster would be "impossible to implement," Russian Foreign Minister Sergei Lavrov was quoted as saying by Russian media.
"In the face of spiralling carnage, Russia and China have shamefully paralyzed the Security Council for far too long," said Richard Dicker, an international justice expert for Human Rights Watch.
He said the Swiss petition "an unprecedented act of 'justice diplomacy'."
"A court investigation would strip all sides to the conflict of their sense of impunity, signaling that abuses could land them in a cell in The Hague," Dicker said.