LUANDA, Angola (CNN) -- A special representative of U.N. Secretary- General Kofi Annan arrived in the Angolan capital of Luanda on Monday, following the downing of a second U.N.-chartered aircraft in that country.

U.N. envoy Benon Sevan went to the Luanda headquarters of the U.N. observer mission with messages from Annan for the government of President Eduardo dos Santos and Jonas Savimbi's rebel movement -- the National Union for the Total Independence of Angola, UNITA.

The United Nations has suspended all its aid flights and announced a review of the role of the U.N. mission, after two U.N. planes crashed within less than 10 days. Saturday's crash occurred when a U.N.-chartered plane was shot down over rebel-held territory. The first plane crashed on December 26, and may also have been shot down. The fate of the 22 people aboard those planes is still unclear.

The United Nations has suspended aid flights to Angola and is reviewing its mission, which has been to monitor a 1994 cease-fire that has now been shattered by a massive resurgence of fighting between government troops and UNITA rebels.

Sevan's efforts in Luanda were expected to focus on getting rescue teams as soon as possible to Vila Nova, the strategic town where wreckage of the first plane has been spotted.

However, fighting in the region is making such a mission very difficult. Vila Nova lies north of Huambo and Kuito, the two central provincial capitals that have been under attack by the rebels for weeks.

The army said on Monday that at least 730 UNITA rebels had been killed in a government offensive on the two cities last month.

There has been no independent confirmation of the casualty figures.

Air traffic to Kuito reportedly resumed on Sunday although only military planes landed at the airport.

The United Nation's chief representative in Angola, Issa Diallo, said Sunday the U.N. monitoring mission had no choice but to recall its monitors from the battle zones in light of the escalating fighting.

A humanitarian worker said Monday that the U.N.'s decision placed Huambo's population of at least 150,000, and a further 100,000 refugees in the city, at risk.

"If the air situation is broken, it means that increasingly in the next weeks or months...the whole population will be vulnerable," the worker told Reuters.

Aid workers expressed fears that the renewed civil war could lead to a situation similar to the one in 1993 and 1994, when UNITA forces besieged government towns for extended periods, leading to mass starvation and disease.

Reuters contributed to this report.