Close on the heels of his earlier warning of pulling out DMK ministers from the Government, Karunanidhi shot off letters to Prime Minister Manmohan Singh and Congress chief Sonia Gandhi, saying he feels "let down" by the "lukewarm" response of the government.
"If our demands are not met, it is doubtful whether our ties with the alliance (UPA) will continue...It won't continue is sure," Karunanidhi told reporters here.
Addressing a hurriedly convened press meet here, the 88-year-old leader said irrespective of the US accepting India's amendments or not, New Delhi should move them at the UN human rights body in Geneva.
The US-sponsored motion puts Sri Lanka on the mat over alleged war crimes and human rights violations against Tamil civilians during the last phase of the war against the Liberation Tigers of Tamil Eelam (LTTE).
The DMK wants the resolution to be stronger by incorporating the demand for an international probe and time-bound action against those found guilty of war crimes in the island nation.
The UN body had adopted a similar US-backed resolution last year with the support of India which asked Colombo to make rapid progress on reconciliation with the ethnic Tamils.
To a question, Karunanidhi said none from the Centre had contacted him after he had warned of his party quitting the government.
DMK, an ally of UPA since 2004, has 18 members in the Lok Sabha with one cabinet minister and four junior ministers.
Seeking to reach out to the DMK, Finance Minister P Chidambaram said India should support the US resolution against Lanka if it called for an independent and reliable probe into the alleged war crimes in the island nation in 2009.