By Yassin Kombi and Sonia Rolley GOMA, Democratic Republic of Congo (Reuters) -Rebels seized the airport of east Congo's largest city Goma on Tuesday, potentially cutting off the main route for aid to reach hundreds of thousands of displaced people,

After 3 UN peacekeepers were killed in eastern Congo, Guterres emphasised that attacks against UN staff may constitute a war crime United Nations chief Antonio Guterres called on Sunday on Rwandan forces to withdraw from Democratic Republic of Congo (DRC) and halt support for fighters advancing on the key Congolese city of Goma.

GOMA, Democratic Republic of Congo (Reuters) -Rwandan-backed rebels marched into east Congo's largest city Goma on Monday and troops from the east African neighbours exchanged fire over the border in the worst escalation of the long-running conflict for more than a decade.

M23 rebels said they have advanced into Goma, a large city in the eastern Democratic Republic of Congo. Hundreds of thousands of residents have been displaced.

M23 rebels in the Democratic Republic of Congo were reported to have taken the key eastern city of Goma in fierce fighting with government forces that sent streams of refugees fleeing area communities.

United Nations chief Antonio Guterres on Sunday urged Rwanda to withdraw its forces from the eastern Democratic Republic of Congo where fierce fighting raged as Kigali-backed fighters closed in on the major city of Goma.

Gunfire broke out before dawn Monday in the centre of the besieged Congolese city of Goma, after the Democratic Republic of Congo accused neighbouring Rwanda of sending more troops across the border to help allied fighters capture the provincial capital.

Congolese rebels and allied Rwandan forces entered the outskirts of Goma on Sunday and the airport in the key eastern city was no longer in use, the top U.N. official in the country said. Thousands fled the fighting that also killed at least 13 peacekeepers.

United Nations chief Antonio Guterres, meanwhile, called on Sunday on Rwandan forces to withdraw from Congo and halt support for fighters advancing on the key Congolese city of Goma.

M23 rebels, reportedly backed by Rwanda, claimed control of Goma, the largest city in eastern Congo, early Monday. This development comes as the Congolese government cut diplomatic ties with Rwanda, with Congolese spokesman Patrick Muyaya calling the situation a "declaration of war.

Fighting with M23 rebels in eastern Democratic Republic of Congo left six United Nations peacekeepers dead, UN officials said Saturday.

The DR Congo army and M23 fighters clashed outside Goma on Friday as the UK, United States and France urged citizens to leave the main city in the country's volatile east, warning the situation could deteriorate rapidly.