(CNN) -- An "ongoing attempted coup d'état" was taking place Thursday in the Republic of Niger, according to the Niger Embassy in London, England.
"Reports reaching us suggest that both the president and the Cabinet ministers who were with him at the time are safe and well," the statement said.
Other media reports, however, said President Mamadou Tandja was missing.
Aid worker Dana Palade with World Vision said she heard the sounds of gunfire, and reported that the streets were deserted.
"The sounds were quite frightening, but the streets are calm. The people are calm," she said, adding the atmosphere was "not what you'd expect in a capital city where you have a coup d'état."
The French Embassy also reported hearing intermittent gunfire near the palace, about 1 kilometer (less than a mile) away.
Resident and activist Laoual Sayabou told CNN the military surrounded the palace, where a ministerial meeting was taking place, about 1 p.m. The military entered the meeting and shots were fired, he said, citing sources in the presidential guard.
"Indications are, it could be an attempted coup," Assistant U.S. Secretary of State P.J. Crowley told reporters. "There was evidently an attempt at assassination of President Tandja."
The U.S. Embassy was monitoring the situation, he said, and embassy staff were safe. Rep. Alan Grayson, D-Florida, is in Niger, he said, and was also safe at the embassy.
Tensions have been rising in Niger recently. Tandja, who has been in office since December 1999, has recently been trying to force through a bid for a third term.
"This is a difficult situation," Crowley said. "President Tandja has been trying to extend his mandate in office." The United States has expressed concern about that, he said, and "that may well have been ... an act on his behalf that precipitated the act today."
While the United States does not condone violence, "clearly we think this underscores that Niger needs to move ahead with the elections and the formation of a new government."
Although Niger is one of the poorest countries in Africa, it has about 8 percent of the world's uranium, and has had some lucrative uranium contracts, particularly with China, CNN's Christian Purefoy said.