Sri Lanka's parliament on Thursday named an ethnic minority Tamil politician as the main opposition leader for the first time in 32 years, a sign of growing reconciliation after a prolonged civil war.

Most of the nation's population belongs to the Sinhalese community. The minority Tamils have alleged persecution by the government since the uprising of Tamil Tiger separatists three decades ago.

Rajavarothiam Sampanthan, 83, the head of Tamil National Alliance, is the first ethnic minority opposition leader since 1983, when Tamil legislators resigned en masse to protest against a law that compelled them to denounce separatism.

Parliament's speaker accepted Sampanthan as the main opposition leader after loyalists to former president Mahinda Rajapaksa were divided on whether they should support the government or go into opposition.

"We will oppose the government on all issues, where it is in the national interest to do so," Sampanthan told parliament in his debut speech as opposition leader.

"We will support the government on all issues, where such support is justified."

Sampanthan is a lawyer who was first elected to the parliament in 1977. His party, the former political proxy of the Tamil Tiger insurgents, backed Maithripala Sirisena in the January presidential elections, defeating Rajapaksa, who ordered the offensive that ended the Tamil insurgency in 2009.

The previous government refused to acknowledge Tamils' request to investigate alleged war crimes during the final phase of the war. The United Nations last year passed a resolution calling for an international inquiry into the alleged human rights abuses.

The outcome of that investigation will be released at the U.N. Human Rights Council session later this month.

The new Sri Lankan government has agreed to a domestic war crimes inquiry and the United States, which sponsored three successive U.N. resolutions against Sri Lanka, said last week it would support a domestic process if it is credible.

Later the parliament approved Prime Minister Ranil Wickremesinghe's proposal to increase the number of ministers in the national unity government to 93 from 70.

The speaker announced that 143 legislators voted in favour and 16 against, while 63 legislators including Rajapaksa and his loyalists were absent.

Sirisena's Sri Lanka Freedom Party (SLFP) and Wickremesinghe's United National Party (UNP), which together account for 85 percent of the 225-member parliament, have agreed to form a national unity government for two years.