Rwanda-backed rebels in eastern Congo claimed Friday to have seized a second airport in the region following a days-long advance, as the U.N. warned that the recent escalation of fighting with government forces has left 350,

Rwanda-backed M23 rebels captured a strategic airport in the Democratic Republic of Congo, expanding their stranglehold over the country’s mineral-rich eastern region.

M23 fighters and Rwandan troops on Wednesday pushed further into the eastern Democratic Republic of Congo, as Burundi warned Kigali it would respond if attacked.

Goma remains “under occupation” by Congo’s M23 rebels and the ceasefire they announced has been broken as their forces engage in heavy fighting along the main route to South Kivu’s main city Bukavu.