(CNN) -- Niger's constitution has been suspended, a Niger military official announced Thursday night on the nation's three television channels.
The order, read by Col. Goukoye Abdul Karimou, was attributed to the Superior Council for the Restoration of Democracy, which also suspended all institutions and called on the nation for calm and on the international community for support, said Ousman Tudou, a journalist for Radio Afini.
Tudou said President Mamadou Tandja and his ministers were being held in a military camp.
No curfew had been ordered and people were in the streets at 11 p.m., around the time of the announcement, he said.
Dana Palade, a spokeswoman for the nongovernmental organization World Vision, told CNN from the capital city of Niamey that the official made the announcement on Doumial Tele Sahel and RTT.
Palade said she heard gunfire in the streets.
"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."
Earlier Thursday, the Niger Embassy in London, England, reported an "ongoing attempted coup d'etat" was taking place in Niger.
"Details remain sparse, but sporadic gunshots have continued to be heard in and around the presidential palace for some time," said a statement from the embassy. "Reports reaching us suggest that both the president and the cabinet ministers who were with him at the time are safe and well."
Other media reports, however, said Tandja was missing.
The French Embassy also reported hearing intermittent gunfire about 1 km (0.6 miles) from the palace.
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.
CNN's Elise Labott contributed to this report.