A UN convoy delivering food into Sudan's al-Fashir in North Darfur came under attack overnight, a spokesperson for the UN children's agency told Reuters on Tuesday, adding that initial reports indicated "multiple casualties".

"We have received information about a convoy with WFP and UNICEF trucks being attacked last night while positioned in Al Koma, North Darfur, waiting for approval to proceed to al-Fashir," a UNICEF spokesperson said in response to questions. She did not say who was responsible.

Famine conditions have previously been reported in al-Fashir, the capital of North Darfur, as two years of war has cut off supplies.

The number of people who have fled Sudan since the beginning of the war has surpassed 4 million, a spokesperson for the UN refugee agency said on Tuesday.

UNHCR spokesperson Eujin Byun told a Geneva press briefing that the milestone was reached on Monday and that the scale of displacement was "putting regional and global stability at stake".