(CNN) Gunfire erupted in Burundi on Thursday, hours after an army general announced a coup and the President's spokesman dismissed it as a "joke."

Political infighting sparked fears that Burundi would plunge into ethnic violence -- with the central African nation's history of civil war making it especially vulnerable to deep divisions.

Sporadic gunshots rocked Bujumbura, the capital, following an attempt to overthrow President Pierre Nkurunziza on Wednesday.

But the President downplayed any coup attempt, urging citizens not to panic.

"We ask all the people of Burundi to stay calm in the face of the impostor," Nkurunziza tweeted. "The situation is under control and the constitutional order has been safeguarded."

Nkurunziza posted a Twitter message later Thursday saying he had returned to his country. It was unclear exactly where he was, but he congratulated the army and the police for "their patriotism," and "Burundians "for their patience."

'Nkurunziza remains the President'

U.S. diplomats expressed concerns at the situation.

"Our embassy has received reports that the airport continues to be closed and that the land borders may also be closed or restricted at this time," said Jeff Rathke, acting spokesman for the State Department.

He told reporters that "President Nkurunziza remains the President of Burundi," but urged Americans there to be cautious. He also said that travel in Bujumbura is not safe.

The United Nations Security Council condemned the violence in Burundi and "called for the swift return of the rule of law," the French mission at the U.N. said Thursday via Twitter.

The Security Council condemns the violent unrest in #Burundi and calls for the swift return of the rule of law pic.twitter.com/FT7oLbjcbA — La France à l'ONU (@franceonu) May 14, 2015

The council, which held urgent consultations on the crisis, also called for credible elections.

'Either it is a coup or not, no one knows'

For a while Thursday, fear and uncertainty reigned as the President's whereabouts were unknown.

Nkurunziza had joined a summit of African leaders in nearby Tanzania on Wednesday to discuss the escalating chaos in his nation.

"People are staying indoors, not moving," said Gad Ngajimana, who lives in Bujumbura. "There was some fighting this morning. Gunfire lasted about 30 minutes. Now there is only gunfire about once every 10 minutes."

He said it was unclear who's in charge.

"The faces of the people -- they are very scared," he said. "Either it is a coup or not, no one knows."

Burundi, like its neighbor Rwanda, has a Hutu majority and a Tutsi minority.

While the current crisis is rooted in politics, some observers fear the government might try to stoke ethnic animosities in a last-ditch effort to retain power.

The last time the nation plunged into ethnic violence fueled by tensions between ethnic Hutus and Tutsis, the resulting civil war left 300,000 people dead.

Refugees flee to neighboring nations

Nkurunziza has been seeking a third term in office, which is prohibited by the agreement that ended the 1993-2003 civil war. Protesters determined to prevent his candidacy have demonstrated in the capital, and have been met with deadly force by police.

About 70,000 refugees have streamed into the Democratic Republic of Congo, Tanzania and Rwanda, according to the United Nations.

The situation can escalate quickly, said Nsengiyumva Pierre Claver, a former member of a European Union electoral monitoring team.

"There is a very great risk of ethnic conflict," said Claver, who is in Bujumbura.

Postpone elections, neighbors say

Animosity against the President started last month when he expressed his intention to run for a third term.

Deadly protests hit the nation as he sought to extend his 10-year rule. He registered last week to run for a third term, angering protesters who've taken to the streets.

Burundi's constitutional court ruled he is eligible to run again because he was picked by parliament, not elected by people, during his first term.

At least seven candidates have registered for the presidential race scheduled for next month. Among them is prominent opposition leader Agathon Rwasa, who helped lead rebel fighters in the country's civil war.

East African Community leaders called for the elections to be postponed, saying conditions are "not conducive" to hold them.

JUST WATCHED Controvery over Burundi President's third term bid Replay More Videos ... MUST WATCH Controvery over Burundi President's third term bid 04:22

Attempted coup

Army Gen. Godefroid Niyombareh announced on the radio Wednesday that the President had been dismissed.

Niyombareh, a former head of Burundian intelligence, was fired by the President in February.

An African Union official said a military coup attempt was underway Wednesday, but the government denied it was under threat.

Reports of a coup are "a joke," government spokesman Willy Nyamitwe said. The President's office said some soldiers had declared an "imaginary" coup. It appealed for calm, saying security forces are looking for the culprits.

"I would call this an attempted coup -- we don't know whether or not it has been successful," said Tim Stevens, a professor at King's College in London and an expert on political violence.

"What we have heard is that there has been both fighting and negotiations overnight, it is still very unclear."

Some airlines, including Kenya Airways, canceled their flights to Bujumbura until further notice.

Burundi is a tiny landlocked nation that is home to about 10 million people.