The US has written to Israel, giving it 30 days to boost humanitarian aid access in Gaza or risk having some US military assistance cut off.

The letter, sent on Sunday, amounts to the strongest known written warning from the US to its ally and comes amid a new Israeli offensive in northern Gaza that has reportedly caused a large number of civilian casualties.

It says the US has deep concerns about the deteriorating humanitarian situation, adding that Israel denied or impeded nearly 90% of humanitarian movements between the north and south last month.

There was no comment from Israel, which says it is targeting Hamas operatives in the north and not stopping the entry of humanitarian aid.