WASHINGTON/TIJUANA (Mexico) • Mexico's Foreign Ministry has presented a diplomatic note to the US government, calling for "a full investigation" into what it described as non-lethal weapons directed towards Mexican territory on Sunday, a statement from the ministry said.
The formal request on Monday came a day after the US authorities fired tear gas canisters towards migrants in Mexico - near the border crossing separating Tijuana from San Diego, California - when some rushed through border fencing into the United States.
More than 40 were arrested on the US side, the US border authorities said, adding that none were believed to have successfully crossed further into Californian territory.
US President Donald Trump told reporters at an event in Mississippi that he would close the border if migrants "charge" the barrier.
During the melee on Sunday, US authorities shut San Ysidro, the country's busiest border crossing, for several hours.
Sunday's incident was the latest chapter in a saga that has pitted Mr Trump's hardline immigration policies against thousands of migrants who have made their way north through Mexico from impoverished Central American countries where violence is widespread.
Tensions had been growing in Tijuana, and Mr Trump said last Saturday the migrants would have to wait in Mexico until their individual asylum claims were resolved in the US. That would be a significant shift in asylum policy that could keep Central Americans in Mexico for more than a year.
Mr Trump on Monday said Mexico should send the Central Americans, mostly Hondurans, back home.
"Mexico should move the flag waving Migrants, many of whom are stone cold criminals, back to their countries. Do it by plane, do it by bus, do it anyway you want, but they are NOT coming into the U.S.A. We will close the Border permanently if need be. Congress, fund the WALL!" Mr Trump tweeted.
Mexico has been in negotiations with the US over a possible scheme to keep migrants in Mexico while their asylum claims are processed.
The team of Mexican President-elect Andres Manuel Lopez Obrador, who takes office on Saturday, has said no deal has been agreed on the migrants. But officials have hinted they could remain.
"We should be objective, whatever happens they will stay in Mexico," said Mr Alejandro Encinas, an incoming deputy interior minister. "Migrants have rights and we will respect them."
US government agencies defended the response to Sunday's incident at the San Ysidro crossing south of San Diego, California. News pictures showing children fleeing tear gas prompted sharp criticism from some lawmakers and charities.
British aid group Oxfam said the use of tear gas was shameful.
"Images of barefoot children choking on tear gas thrown by US Customs and Border Patrol should shock us to our core," Ms Vicki Gass, senior policy adviser for Central America at Oxfam America, said in a statement.
Democrats and other critics called the use of tear gas an overreaction, and questioned the idea of keeping the migrants in Mexico to make asylum claims there.