LAGOS (Reuters) - Nigeria has advised its citizens against any non-urgent travel to the United States until Washington clarifies its immigration policy, after several incidents in which people with valid visas were denied entry, a presidential aide said on Monday.

Abike Dabiri-Erewa, Senior Special Assistant to the President on Foreign Affairs and Diaspora, said no reasons were given for the cases over recent weeks in which Nigerians with valid multiple-entry U.S. visas had been denied entry and sent back to Nigeria.

The West African country is not among a group of Muslim-majority countries from which President Donald Trump wants to suspend travel to the United States on security grounds. About half of its 180 million inhabitants are Muslim, and half Christian.

Trump is expected to sign a new executive order on Monday to implement the travel ban, after his first attempt in January was blocked in the courts.

Dabiri-Erewa did not make any explicit link to the ban, but said Nigerians without any urgent need should “consider rescheduling their trip until there is clarity on the new immigration policy”.

International demand for travel to the United States over the coming months has flattened out, reflecting uncertainty over Trump’s immigration policy, travel analysis company ForwardKeys said on Monday.