U.N. officials say there has been a sudden fall in the number of Syrian refugees arriving in Jordan, but it's unclear why.

Panos Moumtzis , the top U.N. refugee official for Syria, says the usual exodus of up to about 3,000 refugees daily has "dropped to almost zero" over the past four days.

Moumtzis told reporters Tuesday in Geneva that his agency is asking if "people are not being allowed to move, is it security or something else?"

He says the refugee agency lacks staff on the other side of the border and thus is unable to observe the situation there.

Jordanian officials could not immediately be reached for comment on the U.N. concerns.