BISSAU, Guinea-Bissau (AP) — Mortar rounds could be heard Thursday in the capital of the small, coup-prone nation of Guinea-Bissau as the military sealed off the city’s downtown area and lobbed grenades at the prime minister’s home, according to a diplomat and a military official.

The diplomat said the shooting started after the state radio station signal inexplicably went dead. He said that the whereabouts of the nation’s interim president, who took over after the death in January of the previous leader, was unknown.

A military official, who like the diplomat could not be named because he was not authorized to speak to the media, said that the soldiers had encircled the home of Prime Minister Carlos Gomes Jr. He said that they were attacking the building with grenades. It was not clear if the premier was at home when the shooting started — or where he is now.

The attack comes just weeks before an April 29 presidential runoff, which Gomes was favored to win. He took nearly 49 percent of the first round vote, just shy of the majority he needed to avoid a runoff. Challenger Kumba Yala, a former president who was overthrown in a 2003 coup, came second with about 23 percent. But the future of the runoff vote was uncertain because Yala recently announced he planned to boycott the vote, claiming fraud.

It was unclear if a coup was in progress in this country. Like in previous military takeovers, the heavy firing is coming after the state broadcaster went silent. In neighboring Mali last month, renegade soldiers seized state television just after they stormed the presidential palace. They then gathered in front of the cameras to announce the coup d’etat.

The unexpected violence late Thursday took even seasoned diplomats by surprise. One official spoke by telephone to The Associated Press from his office late Thursday, which he had not been able to leave because of the shooting.

“I am at the office and I am prevented from leaving,” said the diplomat. “The downtown area has been sealed off by the military … I can also tell you that all Guinea-Bissau radio has been taken off the air since 8 p.m. local time and the whereabouts of the prime minister and interim president are unknown.”

The presidential election currently in progress was organized in haste, after the death of former leader Malam Bacai Sanha, who died in January after being rushed to France for end-stage diabetes.

Guinea-Bissau has weathered successive coups, attempted coups and a civil war since winning independence from Portugal in 1974. It has been further destabilized by a growing cocaine trade, fueled by traffickers from Latin America who discovered the nation’s archipelago of uninhabited islands several years ago. They used the deserted islands to land small, twin-engine planes loaded with drugs, which are then parceled out and carried north for sale in Europe.

The traffickers, according to analysts, have bought off key members of the government and the military, creating what some are now calling a narcostate.