* Burundi facing worst political crisis since end of war
* Tutsi-led party quit coalition after president sacked VP
* U.N. on collision course with government over U.N. mission
By Patrick Nduwimana
BUJUMBURA, Feb 12 (Reuters) - Burundi's junior coalition party has accused the president of undermining a delicate power-sharing deal, a constitutional requirement that has kept ethnic tensions in check since a 12-year civil war in the east African nation ended in 2005.
The Tutsi-led Uprona party's three ministers quit the coalition administration last week after President Pierre Nkurunziza, whose CNDD-FDD led by ethnic Hutus is the majority party, sacked his Tutsi vice president, also from Uprona.
The row has centred on constitutional amendments proposed by the president that could allow him a third term and change power-sharing arrangements. Opponents say the steps threaten to marginalise minorities, such as the Tutsis.
The turmoil has triggered the worst political crisis since rebels laid down their arms in Burundi - a landlocked country neighbouring Rwanda where Hutu extremists targeted ethnic Tutsis and moderate Hutus in the 1994 genocide.
Burundi's political standoff has also raised the spectre of more unrest in a region already grappling with violence in eastern Democratic Republic of Congo, South Sudan and the Central African Republic.
Adding to tensions, the U.N. Security Council will decide on Thursday whether to renew the mandate of a U.N. mission tasked with supporting political reforms. That vote may test relations between the government, which wants the mission out, and donors, like the United States, that want it to stay.
The Uprona party said it was committed to staying in government, a step that could temper the crisis. But it says it will not be bullied before presidential and parliamentary elections in 2015.
"It is clear the party in power continues with its project to create tension and disorder within our own party," Evariste Ngayimpenda, a senior Uprona official, told Reuters this week.
The presidency gave no comment despite several requests. But CNDD-FDD officials have said the existing constitution was for the transition and needs to be updated to reflect the changes.
Despite relative calm in recent years, rights groups have reported scores of political killings, intimidation of the opposition and a crackdown on media freedoms since Nkurunziza's re-election in 2010.
"The ruling party underestimates the degree of frustration and anger over its authoritarian leadership within opposition parties and the population," said Julien Nimubona, an Uprona government minister until 2013. "This situation risks plunging the country into fresh unrest or even the return to civil war."
The president, an evangelical Christian popular among rural voters, has not publicly said he will run next year, although senior CNDD-FDD officials argue that he can stand again as his election by lawmakers in 2005 does not count as his first term.
The proposed constitutional amendments would also abolish a requirement which limits Hutus to 60 percent of posts in the government and parliament and guarantees Tutsis the rest.
FRUSTRATIONS ON THE STREET
That would upset a balance that has begun healing divisions behind the conflict between the Hutu majority and the Tutsis, who make up about 15 percent of Burundi's 10 million people.
For now, there are no signs tensions have extended into the army, whose leadership is split between Hutus and Tutsis.
"That security pillar of power-sharing in Burundi seems to stand the test so far," said Stef Vandeginste at the University of Antwerp. "If the political tensions translate into serious disagreement and breakdown of trust within the security forces then there will be every reason to be really worried."
Yet on the streets of the low-rise capital, Bujumbura, which lies on the eastern shore of Lake Taganyika, frustrations run high among Burundians who say the government has failed to tackle rampant graft, poverty and high unemployment.
"The CNDD-FDD wants to end the sharing of power between Hutus and Tutsis," said unemployed Jean Claude Ndimwo. "The constitution has to be respected, especially the sharing of power, to avoid chaos."
The landlocked economy, which depends on coffee for half its export earnings, grew about 4 percent annually over the past three years, a rate the World Bank says is insufficient to significantly reduce the high level of grinding poverty.
The June 2015 elections could be a trigger for fresh violence, the U.N. Secretary General Ban Ki-moon noted in his Burundi country report dated Jan. 20.
Bujumbura wants the U.N. mission to leave within six months of its mandate's Feb.15 expiry, arguing it would demonstrate Burundi's political maturity and encourage investment into the world's fourth most aid-dependent country.
Washington has lobbied the government to keep the mission and Ban said a U.N. political presence in Burundi remained necessary in his report before Thursday's Security Council vote.
"It looks really bad," a Western diplomat at the United Nations told Reuters. "(Bujumbura) wants the mission out of Burundi ... and it doesn't' look like they're bluffing."
U.N. officials proposed a scaled-down mission to oversee next year's polls, a compromise the government rejected.
"I think it will probably be renewed, but it won't be for the full 12 months. They want it out before the elections," the diplomat said.
That will worry Nkurunziza's opponents who consider the U.N. presence as vital to fair vote and to contain ethnic tensions.
"We already have serious economic problems," said 32-year-old shop keeper Fatou Kwizera. "Add to these political problems and the situation can only worsen." (Additional reporting by Richard Lough in Nairobi and Lou Charbonneau in New York; Writing by Richard Lough; Editing by Edmund Blair and Andrew Heavens)