COLOMBO (AFP) - Sri Lanka's president on Monday reinstated three key ministers in their posts but delayed announcing a full Cabinet after a landmark agreement to form a coalition government.

Maithripala Sirisena had been expected to swear in a Cabinet on Monday, but official sources said that had been delayed as talks with the Sri Lanka Freedom Party (SLFP) continued.

The SLFP of former president Mahinda Rajapakse agreed on Friday to enter a coalition government with the United National Party (UNP), which won the highest number of seats in parliamentary elections last week.

"There is hectic lobbying going on and that is why the Cabinet (formation) is taking so long," a UNP source told AFP.

Rajapakse, who suffered a shock defeat at the hands of Sirisena in January's presidential election, will sit on the opposition benches along with a dissident group of loyalists.

Former Foreign Minister Mangala Samaraweera got his old job back, as did Justice Minister Wijeyadasa Rajapakshe and D. M. Swaminathan, minister for resettlement.

All three were required to address issues arising from a UN Human Rights Council report on alleged war crimes in Sri Lanka during the final stages of the war against separatist Tamil Tiger rebels in May 2009.

The report is to be made public at the next session of the UN Human Rights Council next month, but Colombo will be given an advance copy to allow it to respond before then.

Rajapakse's government had refused to cooperate with the UN and insisted that not a single civilian was killed by troops under his command.

The new government, however, has agreed to cooperate with the UN and address allegations that government forces killed at least 40,000 ethnic Tamils while fighting the separatist Liberation Tigers of Tamil Eelam (LTTE).

UN chief Ban Ki Moon urged Sri Lanka's new government over the weekend to ensure "long term peace" after the agreement to form a coalition committed to ethnic reconciliation.