Foreign donor countries have strongly urged the Sri Lankan government not to allow a key coalition partner to undermine peace efforts.
The JVP has led street protests against Norway's role
Japan, the US and the Netherlands say they are concerned by the activities of the JVP which is staunchly opposed to the Norwegian-brokered peace deal.
The three countries co-chair efforts to attract international support to rebuild Sri Lanka.
Peace talks stalled last year following a ceasefire in February 2002.
Ambassadors from the three donor countries met President Chandrika Kumaratunga late on Tuesday as a top Norwegian envoy, Erik Solheim, arrived in Sri Lanka for more talks to try to bolster the peace process.
Mr Solheim travelled to Kilinochchi in the north on Wednesday to see leaders of the Tamil Tiger rebels.
He said after the meeting that the peace process was in "critical" condition.
"There has been a tendency recently that both sides have been acting to irritate the other side rather than to build up
trust," Mr Solheim said.
He will meet President Kumaratunga later in the week.
After meeting the president, the donor representatives issued a statement expressing "deep concern about the ongoing JVP-led actions against the peace process in Sri Lanka and the government of Norway's efforts as facilitators of that process".
It continued: "The representatives expressed bewilderment that a member of the UPFA [coalition government] could engage in such a campaign in absolute contradiction of the clearly stated position of the president and the government."
The Sinhala nationalist JVP are left-wingers who were once involved in some of the worst violence in Sri Lanka's recent history.
They are strongly opposed to Norway's role in the peace effort, and have mounted vocal street protests and repeatedly asked Oslo to quit.
The party opposes any concessions to the Tamil Tiger rebels as part of a deal to revive the peace talks and have accused the Norwegians of being biased towards the rebels.
The Tigers have been fighting for more than two decades for self-government in the north and east, which they consider the Tamil homeland.
More than 60,000 people have been killed in the course of the conflict.