A group of Indian lawmakers, whose support is crucial to the government, handed their resignation letters to their party chief on Friday over the conflict in Sri Lanka, officials said.
The 13 lawmakers, including six ministers of a regional party in Tamil Nadu, want India to ask Sri Lanka to call a ceasefire immediately.
The lawmakers say they share the increasing concern of the mainly Tamil population on the island that Sri Lankan troops are wiping out Tamils there.
Sri Lanka has vowed to crush the Tamil Tigers (LTTE) militarily. It says its troops are 2 km from the rebel capital of Kilinochchi, a strategic and symbolic target.
Troops stepped up the offensive against the LTTE rebels fighting for a separate homeland this year and the government says its forces have killed thousands of rebels since January.
The lawmakers' resignation could force a vote of confidence in Prime Minister Manmohan Singh's government if the letters reach the speaker in India's parliament.
But opposition parties, who have described the move as a "farce", and analysts said the threat may be more about making political noise for their local constituencies.
"They are unlikely to carry out the threat and the government will not face a crisis," C. Uday Bhaskar, a strategic analyst said.
Earlier this week, about 39 lawmakers, all allies of ruling Congress party-led coalition met in Chennai and gave the government two weeks to intervene or face being brought down.
The meeting was chaired by M. Karunanidhi, chief minister of Tamil Nadu and leader of the Dravida Munnetra Kazhagam party (DMK), a key ally of the Congress party.
"We are not speaking for the LTTE, but for the orphaned Tamils," Karunanidhi said on Friday. "I am not asking for armed intervention but only restoration of peace in Sri Lanka."
India's prime minister has urged Sri Lanka to solve the conflict politically but said it was not going to intervene.
India sent peacekeepers to the island nation in 1987, only to withdraw them after losing more than 1,200 men in battle and facing allegations of human rights violations.
"I am sure Mr Karunanidhi will understand that our prime minister has come out in support of innocent Tamils caught in the battle," Veerappa Moily, a senior Congress party spokesman said in New Delhi. "This issue will be resolved."