By Andrew Heavens

KHARTOUM, Oct 23 (Reuters) - The U.N.'s new top envoy to Sudan was due arrive in Khartoum on Tuesday, a year to the day after his predecessor was expelled.

Pakistani diplomat Ashraf Qazi will take up his duties as the U.N. Secretary General's special representative in Sudan at a time of deepening political uncertainty in Africa's biggest state.

A faltering north-south peace deal, which Qazi will be expected to oversee, was thrown into crisis when former southern rebels withdrew their ministers from the country's coalition government on Oct. 11.

Qazi's arrival also coincides with an upsurge in violence in Sudan's western Darfur region, just days ahead of peace talks between the government and splintered rebel groups.

It was Darfur that sealed the fate of Qazi's predecessor, the Dutch diplomat Jan Pronk. Pronk angered Khartoum by publishing comments on his weblog saying the Sudanese army had lost two major battles to rebels in North Darfur and soldiers were refusing to fight.

He flew out of Khartoum on Oct. 23, 2006, after the government ordered him to leave, calling him a threat to Sudanese security.

U.N. officials in Khartoum said the timing of Qazi's arrival was a coincidence.

"It is just that Mr Qazi had some work to finish up in his previous mission before arriving here and this is how the timing worked out," said spokeswoman Radhia Achouri.

Qazi was the U.N.'s special representative in Iraq before taking up the Sudan post.

U.N. staff are known to be unhappy about the length of time it has taken to replace Pronk. The appointment was held up by the handover from Kofi Annan to the current U.N. Secretary General Ban Ki-moon.

Qazi is due to make is first public appearance at U.N. Day celebrations on Wednesday.

Qazi told Reuters in September that his first job in Sudan would be to win the trust of the signatories of the Comprehensive Peace Agreement - a deal that ended a two-decade long civil war between the north and the south.

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