NAIROBI — President Trump's travel ban, which goes into effect Thursday night, will temporarily bar refugees and some foreign visitors over national security concerns. Other countries are also imposing new restrictions on refugees — in extreme cases, even physically forcing them back over the border.

The United Nations reported Thursday that nearly 900 Nigerian refugees — mostly children — were rounded up in Cameroon this week by the Nigerian military and the Cameroonian police. They were sent in trucks back over the border to a Nigerian city that lacks food and water and where many homes have been bombed into rubble. This was the latest example of the “forced returns” of thousands of refugees who in recent years had fled from Boko Haram's offensive in northeastern Nigeria, in which the Islamist extremist group killed, raped and kidnapped thousands of people, according to the United Nations and aid agencies.

In April, I visited some of the refugees who had been rounded up at gunpoint in Cameroon and returned to the bombed-out Nigerian city of Banki. (Here's my story). This happened even after U.N. officials reached an agreement with the Nigerian and Cameroonian governments, mandating that refugees return only voluntarily. That agreement has been violated multiple times, according to U.N. officials, and Tuesday's deportation marks one of the starkest violations.

“The involuntary return of refugees must be avoided under any circumstances,” U.N. High Commissioner for Refugees Filippo Grandi said in a statement. “In addition, returns to Nigeria put a strain on the few existing services and are not sustainable at this time. A new emergency, just as the rainy season is starting, has to be avoided at all costs.”

Cameroon is among 145 countries that have ratified the 1951 Refugee Convention, which stipulates that victims of war or persecution should not be returned to nations where they will face serious threats.

The Cameroonian government has in the past denied that such forced returns had taken place. A spokesman for the Cameroonian government could not be reached by phone Thursday. Some Nigerian authorities have suggested that their country is prepared for refugees to return, despite warnings from aid agencies.

Human rights groups have alleged that refugees are also being forced or pressured to return from Pakistan to Afghanistan, from Turkey to Syria and from Kenya to Somalia. The host governments have denied such allegations.

“Poorer countries hosting huge numbers of refugees for many years, such as Kenya, Pakistan and Turkey, have recently pushed back hundreds of thousands of refugees and asylum seekers,” Gerry Simpson, a migration expert at Human Rights Watch, told me recently. “They seem to be taking their lead from richer countries, such as Australia, the E.U. and the U.S., who are pulling out all the stops to limit refugee arrivals.”

Banki is one of a number of Nigerian cities that have basically been transformed into open-air displacement camps, many of them suffering severe food shortages. Banki has more than 45,000 internally displaced people, who, according to the U.N. statement, “are already barely accommodated.”

The United Nations described “conditions of severe overcrowding and without basic facilities such as drinking water, sanitation and health facilities.” Banki also still faces a threat from Boko Haram militants.

The United Nations also has brokered “tripartite agreements” to protect Somali refugees in Kenya and Afghan refugees in Pakistan from being pressured or forced to return to their war-scarred countries. In both cases, human rights groups have said that those agreements have fallen short.

U.N. officials said they are trying to persuade the Cameroonian government to respect the right of Nigerian refugees to seek shelter across the border. But the officials have said privately that such engagements have been tense and that their guidance has often been ignored.

Some Cameroonians have claimed that Nigerians are consuming their country's scare resources. Relief officials also suspect that the Cameroonian military might be moving refugees ahead of military operations aimed at Boko Haram.