Sirisena had replaced him with former strongman Mahinda Rajapaksa and suspended Parliament until November 16 in an apparent attempt to give Rajapaksa time to muster enough support to survive any no-confidence vote.
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.
Al Jazeera's Bernard Smith reports from Colombo.
The president's move has plunged the country into a constitutional crisis with Wickremesinghe claiming he is still the prime minister.
The minority Sri Lanka Muslim Congress party, which has seven lawmakers, said its members had also rejected offers to join the Sirisena-Rajapakse camp.
On Friday, Sirisena had dismissed Prime Minister Ranil Wickremesinghe and his Cabinet, later telling reporters he acted in part because Wickremesinghe and a Cabinet colleague were behind an alleged assassination plot against him.
Besides domestic politics, strategic experts view the political developments in Sri Lanka as the playing out of geostrategic rivalry between India and China.
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After winning the presidency as a neutral candidate in 2015, Sirisena accepted an offer from Rajapaksa to take over his Sri Lanka Freedom Party.
Wickremesinghe survived a no-confidence motion in Parliament in April brought by Rajapaksa supporters over allegations involving his appointment of a Singaporean as the central bank governor, who is now accused of leaking inside information to benefit his son-in-law in a treasury bond sale.
However, there has been no statement from President Sirisena or Prime Minister Rajapakse since then.
Asked what his next step would be if Parliament is not convened before 16 November, Wickremesinghe said that they had a plan in mind and would implement it. Thousands of Sri Lankans also protested in the capital in the past week demanding Sirisena immediately convene Parliament.
The announcement was made by Parliament spokesman Karu Jayasuriya when he addressed a special meeting of party leaders.
Wickremensinghe tweeted saying "the people's voices have been heard".
Prior to the crisis, Wickramasinghe's UNP had the backing of 106 parliamentarians while Rajapaksa and Sirisena combine had 95 seats.