The PM said that it is India's conviction that a meaningful devolution package would lead towards a lasting political solution.
"I had instructed our delegation to remain in close contact with its Sri Lanka counterparts in an attempt to find a positive way forward. Your Excellency would be aware that we spared no effort and were successful in introducing an element of balance in the language of the resolution," Singh said.
He was referring to India voting against Sri Lanka at the UNHCR in Geneva when it supported the resolution against alleged human rights violations during the civil war. Though India voted in favour of the resolution, it worked behind the scenes to tweak the document to make it "non-intrusive".
Singh's letter comes against the backdrop of Sri Lanka's displeasure over India voting against it at the UN and crticisim from various sections that "domestic compulsions" in had forced the country to vote against it.
In the letter, which suggests that Rajapaksa had written to him three days before the vote, Singh noted that both countries had discussed the way ahead to a political solution that will address all outstanding issues, in particular the grievances of the Tamil community in Sri Lanka, in a spirit of understanding and mutual accommodation.
"It is our conviction that a meaningful devolution package, building upon the 13th Amendment, would lead towards a lasting political settlement on many of these issues and create conditions in which all citizens of Sri Lanka, irrespective of their ethnicity, can find justice, dignity, equality and self-respect," Singh said.
Earlier, Sri Lankan President Mahinda Rajapaksa had warned that countries which voted for the resolution will have to worry about consequences of terrorism.
The president commended the 15 countries which voted against "the anti-Lanka resolution for their support" and the eight nations which abstained from the vote in the 47-member UN Human Rights Council.
India was one of 24 nations which voted for the resolution, but after ensuring critical amendments that forbade intrusion into Sri Lankan affairs by the United Nations or others.
India forced the US to amend its resolution to safeguard Sri Lanka from international intrusions by adding a clause that said the UNHRC can provide advice and technical assistance "in consultation with, and with the concurrence of, the government of Sri Lanka".
Sri Lanka downplayed India's support to the resolution, saying New Delhi gave in to pressures from its coalition partners in Tamil Nadu ranged against Colombo.