Tanzania's President Jakaya Kikwete is due in Kenya to try to salvage deadlocked talks between the government and opposition.
Mr Annan (centre) has had little luck in prodding the rivals towards a deal
President Kikwete, who is also African Union chairman, arrives a day after mediator Kofi Annan expressed his frustration at the lack of progress.
The talks stalled after the government changed its position on the question of power-sharing.
Police say at least 1,500 people have been killed since the disputed poll.
Opposition Orange Democratic Movement (ODM) Raila Odinga says he was cheated of victory by President Mwai Kibaki.
Tension is high as the says it will hold mass protests on Thursday unless parliament is reconvened to amend the constitution to pave the way for power-sharing.
In Kitale, in the west of the country, police have arrested some 200 youths believed to be undergoing military training on a private farm.
Violence has continued since the December elections
"We arrested the youth and recovered wooden carved guns but are still searching for real weapons. The boys say they were being trained as security guards," senior regional police investigator Issa Mohammed told the BBC.
Mr Issa said investigations were still ongoing to verify claims that militia groups were being formed in readiness for war if the mediation talks fail.
In Nairobi, Mr Annan held separate talks with both negotiating teams before the joint sessions resumed.
The former UN secretary general met both Mr Odinga and Mr Kibaki on Monday to urge them to reach an agreement.
Prisoner of peace
Mr Annan has been in Kenya for more than a month trying to resolve the crisis - the longest period he has spent on any conflict resolution.
Afterwards he said the mediation team had "done its work - I'm now asking the party leaders to do theirs".
Mr Annan is reported to have said that he feels like a prisoner of peace - unable to achieve an agreement but unable to leave Kenya.
Last week, both sides agreed to create the post of prime minister, which would be taken by Mr Odinga, leading to hopes of a final deal soon.
However, they still needed to finalise which powers he would have.
The government now says the president should appoint the prime minister, which would not be an executive post.
As well as how to divide powers between a prime minister and a president, the rivals are also split on sharing on cabinet positions and the possibility of a new election if the coalition collapses.