Amid the ongoing political tension, Resident Coordinator of the United Nations in Sri Lanka Hanaa Singer has met Sirisena and Parliament Speaker Karu Jayasuriya, delivering Guterres' message for the need to respect democratic values and constitutional provisions.
It has two prime ministers, or at least two politicians both laying claim to be the sole legal head of government.
The TNA has still said it will back a motion against Rajapakse that the UNP has submitted to be taken up when parliament does reconvene.
Most of the 22 remaining MPs, including the TNA, are likely to oppose Rajapaksa, experts said.
In a statement, the TNA Saturday said Rajapaksa's appointment was a violation of the Constitution's 19th amendment.
Meanwhile, thousands of supporters continue to occupy Wickremesinghe's official residence saying they would resist any move by Sirisena to evict their prime minister.
Huge amounts are reportedly being offered to defectors.
The minority Sri Lanka Muslim Congress party, which has seven lawmakers, said its members had also rejected offers to join the Sirisena-Rajapakse camp.More news: Samsung might remove Headphone jack from Galaxy S10
"We appeal to you not to allow parliament to be put up for sale".
Rajapaksa was Sri Lanka's president from 2005 to 2015.
It expressed alarm at ministerial positions being given for changing political loyalties. The subsequent appointment of Mr Mahinda Rajapaksa as Prime Minister has led to a stable alliance which includes several lawmakers from the former PM's United National Party.
Wickremesinghe said in an interview with The Associated Press that there is credible evidence that Rajapaksa's party is attempting to buy support in Parliament. Sirisena has been sending conflicting signals about his willingness to end the parliamentary suspension.
The announcement was made by Parliament spokesman Karu Jayasuriya when he addressed a special meeting of party leaders.
The Commonwealth Secretary General Patricia Scotland called on the political leadership and national instituitions to uphold the constitution.
Former strongman, president Mahinda Rajapaksa, was sworn in as the new prime minister and Parliament was suspended, sowing economic and political uncertainty, in the South Asian island nation.