LUANDA, Angola (CNN) -- A second U.N.-chartered aircraft was shot down in the embattled central region of Angola on Saturday, the second such incident in a week, the United Nations observer mission said.

A U.N. spokesman said the aircraft had been evacuating seven U.N. workers from Huambo, a central city under rebel siege, to the capital Luanda when it was downed around 3 p.m. (1400 gmt).

Angolan state radio said the plane was hit near Bailundo, a UNITA stronghold some 80 km (50 miles) north of Huambo and the fate of those on board was not known. "The aircraft tried in vain to make an emergency landing at Huambo airport," it said.

Officials of the UNITA rebel movement were not immediately available for comment.

A week before, a U.N.-chartered C130 transport plane with 14 people aboard was shot down near Huambo.

U.N. spokesman Amadou Toure said Saturday's plane had been flying U.N. workers out of Huambo despite fierce fighting in the area.

"We are relocating our people. How else are we supposed to do it. By boat?" Toure told Reuters.

Huambo is several hundred km (miles) from Angola's Atlantic seaboard.

The U.S. State Department released a statement on the crash Saturday afternoon.

"The United States is shocked and saddened by the loss of a second United Nations Observer Mission in Angola (MONUA) plane in one week. A UN-chartered aircraft reportedly went down over UNITA-held territory on January 2 with eight passengers and crew. While the final passenger list is not confirmed, we are aware of at least one American citizen aboard. The fate of the passengers and crew is uncertain, since the crash site has not yet been reached.

The statement goes on to insist that the UNITA rebels provide "immediate, full cooperation" in the search and rescue efforts.

The statement concludes, "we remain convinced that only a negotiated settlement can bring peace to Angola."

Contradictory reports clouded the fate of the 14 crew and passengers of another U.N.-chartered C130 aircraft downed near Huambo on December 26.

UNITA on Saturday denied government reports that it was holding survivors from the South African-owned aircraft.

A spokesman for U.N. observers in Angola quoted government officials as saying that all 14 people aboard that plane were alive, but the United Nations had not confirmed this.

The wreckage has been spotted by high-altitude aircraft but no one has been allowed near the site-- in an area presumed to be under the control of UNITA, or the National Union for the Total Independence of Angola.

UNITA Secretary-General Paulo Gato told Reuters that UNITA was not holding any survivors, nor was it responsible for shooting down the plane. "The only plane we have shot down is the MiG," Gato told Reuters via telephone from central Angola.

He was referring to UNITA claims that its rebels had shot down a government MiG-23 fighter aircraft over Huambo on Friday.

A defence ministry source in the Angolan capital denied the MiG was shot down. "It was an emergency landing (at Huambo airport) due to mechanical failure. The UNITA claims do not correspond to the true facts," the source told Reuters.

Angolan army spokesman Brigadier Jose Manuel Jota told state radio he was informed that the survivors "are under (UNITA leader) Jonas Savimbi's custody somewhere in the Andulo or Bailundo areas."

Fighting around Huambo and nearby has shattered Angola's 1994 peace accord and pointed to a resumption of the civil war that began in conjunction with Angola's move to independence from Portugal in 1975.

Reuters contributed to this report.