NAIROBI/KINSHASA (Reuters) - A change of tone in Angola’s relationship with longtime ally Congo has left Congolese President Joseph Kabila more isolated than ever as he clings to power in his vast central African country.

FILE PHOTO: Democratic Republic of the Congo's President Joseph Kabila meets some members of the opposition parties inside his office at the Palais de la Nation building in Gombe municipality in the capital Kinshasa, Democratic Republic of the Congo April 3, 2017. REUTERS/Kenny Katombe

Angola, a regional political and military heavyweight, has on several occasions provided vital support for Kabila, who took over as president of Democratic Republic of Congo in 2001 following the assassination of his father.

But Luanda is frustrated by Kabila’s handling of several crises, including his failure to step aside when his mandate ended last December and a conflict in which refugees have poured across his country’s long border into Angola.

In December Angola withdrew military trainers it had sent to Congo, where wars at the turn of the century killed millions and sucked in neighboring countries.

That decision raised doubts that Luanda would be ready to bail Kabila out again, and those doubts have been heightened by the change of tone in ties, signaling a big shift in regional power politics.

Foreign Minister Georges Chikoti made clear that patience was running out in May, when he questioned a statement in which Congo said a conflict in the Kasai region bordering Angola had been resolved after several months.

“On one side, we are told that the question of the succession of the kingdom of Kasai has been resolved, on the other, we (still) see people who arrive (in Angola) having been treated badly,” he told French broadcaster RFI.

Chikoti also backed calls for an international inquiry into the killing of two U.N. investigators in the region, a move which Kinshasa has rejected.

“PLAYING WITH MATCHES ON A BARREL OF EXPLOSIVES”

A more direct challenge has come from Sindika Dokolo, a Congolese businessman and art collector who is married to the billionaire daughter of President Jose Eduardo dos Santos.

Having previously steered clear of national politics, Dokolo has in recent weeks hit out at Kabila on Twitter and in interviews, drawing comparisons with the end of Congolese late dictator Mobutu Sese Seko’s rule.

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“We underestimate Congo’s capacity to destabilize the region,” he told Reuters. “We are playing with matches on a barrel of explosives and that worries me a lot.”

Dokolo has urged students and church leaders to mobilize against Kabila and praised Moise Katumbi, an opposition leader in exile.

He says he is commenting as a private Congolese citizen. But the outspoken remarks carry weight because they come from the heart of the family around dos Santos.

“Clearly as husband to Isabel dos Santos, this signals frustration in Luanda,” said Alex Vines, head of the Africa Program at London-based policy institute Chatham House.

PRESERVING THE STATUS QUO

Congo, a country of more than 80 million people which is the world’s biggest cobalt producer, a copper and diamonds miner and an oil producer, has been plagued by war and instability since the fall of kleptocrat Mobutu in 1997.

Angola has had differences in the past with Congo, including over their maritime border, competing claims to offshore oil and the expulsion by Luanda of tens of thousands of Congolese diamond miners more than a decade ago.

But Angola, whose population of about 26 million is much smaller than its neighbor’s, believes its interests are best served if Congo is stable, though weak, and has stepped in militarily or diplomatically in the past to keep the status quo.

In 1998, early in a war that lasted until 2003, Angolan warplanes strafed Rwandan soldiers marching on Kinshasa. When gun battles erupted in the capital in 2006, Angolan troops flew in to help Kabila’s bodyguards defeat fighters loyal to Jean-Pierre Bemba, an ex-rebel he had just defeated in an election.

Luanda has also echoed Kinshasa by criticizing sanctions imposed on Congo by the United States and the European Union over the election delays and alleged human rights abuses.

But in a sign of Angola’s growing frustration with the situation in Congo, Luanda has -- as a member of the U.N. Security Council in 2015 and 2016 -- voted for tough resolutions against Congo at the United Nations.

Luanda also showed its concern by putting pressure on Kabila to accept a second attempt at dialogue late last year after talks on a political transition, brokered by the African Union, failed to include the main opposition leaders.

“The Angolans don’t respect or appreciate the way Congo is handling the crisis. They feel they are making it worse,” a diplomat in direct contact with top Angolan officials told Reuters. “It is very clear to the Angolans that Kabila doesn’t have control of his country.”

DIPLOMATIC OFFENSIVE

Barnabe Kikaya bin Karubi, Kabila’s top diplomatic adviser, said the Congolese government understood Angola’s frustration about violence near its border but that Angola had not pressured it over internal politics.

Angola’s concerns, and those of other African leaders and Western states, are heightened by the fact that unrest in Congo often takes on regional dimensions.

A repeat of the overt interventions by several neighbors in the 1998-2003 war is unlikely, but Kabila is seeking regional support as the political impasse continues in Congo and has launched a diplomatic offensive to woo African countries.

He began in April by visiting Egypt and Gabon, where joint statements were issued criticizing foreign meddling. Since then, his foreign minister has visited the nine countries neighboring Congo and other African states to deliver letters from Kabila.

Sitting African leaders are reluctant to criticize Kabila publicly, but the Ghanaian former U.N. Secretary-General Kofi Annan and nine former African presidents have said Congo is in “grave danger” because of the failure to organize elections.

Some Congolese fear Angola could intervene militarily as it has troops and weaponry on the border.

But Angola’s economy has been severely weakened by low oil prices -- year-on-year inflation is more than 32 percent -- and Luanda is trying to manage a transition of its own, with dos Santos due to quit in August after 38 years in power. An election is due in Angola on Aug. 23.