Chairman of Samajbadi Forum and Chairman of the Sanghiya Gatbandhan, Upendra Yadav said he was not aware about the meeting.
"I came to know about the meeting convened by PM only after reading today's Kantipur daily. It wouldn't be appropriate to attend the meeting summoned through news report," Yadav said.
"The PM has to make it clear why the meeting was called. He has to send formal invitation. PM Oli's manner of summoning the meeting doesn't seem right," Kathmandu Post quoted him as saying.
Writing a special letter to Gathabandhan leaders last night, Prime Minister Oli had summoned an all-party meeting for this morning to discuss ways to resolve the Madhes crisis.
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All the political parties represented in Legislature- Parliament were invited at the meeting.
Sanghiya Gathabandhan has been organising protests in Kathmandu against the new Constitution.
The Madhesi forces had earlier also snubbed an all-party meeting called by PM Oli.
Speaking to reporters some hours after the all-party meeting, Mahato said the UDMF was ready to sit for talks if the government responded to their demands.
The major demands of Madhesis, mostly of Indian-origin, include re-demarcation of the seven province model of federal structure, inclusiveness and proportionate representation of marginalised groups and ethnic minorities including the Madhesis, indigenous groups and dalits in all the state bodies.
Madhesis earlier had launched six-month-long agitation from September to February in which more than 50 people were killed. The agitation had also crippled the landlocked country's economy as supplies from India were blocked.
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Mahato said the UDMF would formally make a decision to make appropriate response to the Prime Minister's call for talks.
He said he was for amending the Constitution, not rewriting the constitution or forming a new Constituent Assembly.
The ruling parties and the main Opposition Nepali Congress have agreed to address the demands of agitating parties through dialogue and discussion.
During a meeting held today at Singha Darbar secretariat, the ruling parties, including CPN-UML and CPN Maoist Centre, and the main opposition Nepali Congress discussed how to adopt flexibility in addressing the demands of disgruntled parties.
Responding to a query, Minister Kharel said the government had sent a letter in the name of United Democratic Madhesi Front calling for the talks.
Leader of the Maoist Centre, Ram Bahadur Thapa, said although parties associated with Federal Alliance did not attend the meeting, the ruling parties and main opposition party held discussions on the issue of resolving the existing problems.
Deputy Prime Minister and Chairman of the Madhesi Janadhikar Forum (Democratic), Bijay Kumar Gachchhadar, said the Prime Minister had sent the letter to agitating parties for talks expressing determination that the government wished to resolve problems through talks.