Maseru - South African President Jacob Zuma will now travel to Maseru on Tuesday to try rescue the deal agreed just a week ago, diplomatic sources in Lesotho and South Africa said.
Prime Minister Tom Thabane said the actions of an army general - who refuses to step down and has seized army weapons - made parliament's reopening impossible.
"The situation in the country is not stable, how do we re-open parliament under these conditions?" the prime minister told AFP.
"We have a renegade army general who has gone rogue with some weapons from the country's armoury and refuses to submit to authority, this is not the time to re-open parliament."
General Tlali Kamoli is accused of being behind the 30 August putsch, which forced Thabane to flee to South Africa.
Setback to peace deal
Thabane's comments present a serious setback to hopes of peacefully ending the crisis in the small nation surrounded by South Africa.
During last week's talks Thabane appeared to succumb to months of opposition pressure to re-open the legislature, which was closed in June amid fears he would lose a non-confidence vote.
He had agreed to reopen parliament on 19 September.
Thabane's political foes in the Lesotho Congress for Democracy have accused him of already reneging on that deal.
"Mr Zuma will arrive in the country tomorrow, as we reported to [regional bloc] SADC that our prime minister is not adhering to the conditions of the road map," said party member and Communications Minister Selibe Mochoboroane.
"While he may address other issues... I believe the PM's refusal to re-open parliament is the main issue."
The LCD's leader, Deputy Prime Minister Mothetjoa Metsing, is accused of being in league with the army in staging the failed coup.
More robust force
As part of last week's agreement Zuma has deployed South African police to protect Thabane and some of his key allies, but Lesotho's leaders are calling for a more robust force to hunt renegade general Kamoli.
Lesotho's army commander Lieutenant General Maaparankoe Mahao told AFP on Sunday that military action was now the only option against Kamoli.
"Negotiations have failed as far as we are concerned. At this point in time I think we are left with no option but military operations."
The army chief was frank about the difficulties he would face in taking on Kamoli, who has taken to the mountains with a seized cache of weapons including artillery, mortars and small arms.
However, "it would definitely be very helpful" to get military support from the Southern African Development Community, a 15-nation regional bloc.
"I have asked the relevant body in SADC to consider coming to our assistance," Mahao said.