David Cameron is pushing for an independent, international investigation into alleged atrocities during the Sri Lankan civil war to begin within a year as pressure mounts on the regime in Colombo.
The Prime Minister has personally sought the support of several fellow leaders for a United Nations Human Rights Council resolution authorising a UN-led probe.
As many as 40,000 civilians are estimated to have died in the final months of the Sri Lankan government's 26-year fight with Tamil Tiger separatists - which ended in 2009.
Both sides in the brutal conflict are accused of widespread human rights abuses - including executions and rapes of prisoners by state troops.
US ambassador to the UN Samantha Power wrote on Twitter:
The proposed referendum on the future of #Crimea would violate the Ukranian constitution. The U.S. will not recognize its results.
United Nations special envoy John Serry has exclusively told ITV News that his ordeal is an example of how serious the situation has become in Ukraine's Crimea region.
Mr Serry was forced to take refuge in a coffee shop with journalists from ITV News after being confronted by armed men.
"I think this incident is just an example of how serious the situation has become and my hope is that everybody will realise that this situation is very urgent now and we must find a way to de-escalate the situation here," Mr Serry said inside the business lounge at Simeferopol airport.
"That is the lesson of what happened to me."
John Serry says something needs to be done to "de-escalate" the "very urgent" situation in Crimea, after he was forced to leave the region following his confrontation with unidentified armed men and angry pro-Russia protesters.
Mr Serry added that he hoped all citizens in Crimea can "start to feel safe again".
The situation is very urgent now and we need to find a way to de-escalate the tension - UN's Robert Serry tells us before flying out.
UN special representative Robert Serry has said his forced departure from Crimea reflects the seriousness of the situation in the region as he spoke to ITV News' Europe Editor James Mates.
We have a second exclusive interview tonight with UN envoy Robert Serry, inside the business lounge at Simferopol airport. #Crimea
UN's Robert Serry tells us he feels for the people of #Crimea and the fact he's being forced to leave shows how serious the situation is.
ITV News were the only TV journalists with a UN envoy as he was forced to seek refuge in a Crimea cafe after being confronted by armed men.Read the full story ›
Europe Editor James Mates, who has been with the UN's special envoy since he was trapped in a cafe in Crimea, reports that Robert Serry is to get a flight to Turkey:
Local militia blocked a door to a cafe in Crimea trapping UN special envoy Robert Serry and ITV News journalists.
Mr Serry sought refuge in the cafe after his car was blocked by armed men who demanded that he should leave the region.
An aide to UN special envoy Robert Serry asked journalists from ITV News not to leave the coffee shop in Crimea where the diplomat was being held by unidentified local militia.
"No, no don't leave. Please," the woman is heard saying as Mr Serry discusses the situation on the phone.
ITV News were the only TV journalists with the UN envoy as he was trapped in a cafe.