The EU said on Saturday it was "urgently" considering all options in response to a widening rebel offensive in the eastern Democratic Republic of Congo, after Rwandan-backed forces seized a second major city.

Having captured the key provincial capital of Goma last month, M23 fighters pushed south and took a vital airport before marching virtually unchecked into another key city, Bukavu, on Friday, according to security and humanitarian sources.

"Alarmed by news of Rwandan backed M23 forces seizing Kavumu airport and entering Bukavu, ignoring international appeals for ceasefire," European Commission spokesman Anouar El Anouni wrote on X.

"The EU urgently considers all the means at its disposal. The ongoing violation of the DRC's territorial integrity will not go unanswered."

The warning comes as the conflict is set to dominate an African Union summit that opened on Saturday in the Ethiopian capital Addis Ababa with UN chief Antonio Guterres saying a regional escalation "must be avoided at all costs".

It follows an appeal from the European Parliament, which this week urged the EU to suspend a minerals deal with Rwanda in response to Kigali's involvement in the offensive.

Lawmakers in Strasbourg on Thursday overwhelmingly approved a non-binding resolution which also called for the bloc's 27 member states to freeze all direct budget support as well as military and security assistance to Rwanda.

Rwanda denies giving military support to the M23 but has accused extremist Hutu groups in DR Congo of threatening its security.

A report by UN experts said last year Kigali maintained around 4,000 troops in DR Congo and had de facto control of the rebel group.

The conflict has killed thousands and driven vast numbers from their homes. (AFP)