MEXICO CITY - US border agents launched tear gas into Mexico early on Tuesday to deter a group of migrants from crossing the US-Mexico border at Tijuana, according to a Reuters witness and the US government.

Clouds of the noxious gas could be seen wafting up from around the fence at the border, and one migrant picked up a canister and threw it back across the border into US territory.

Tijuana has become a flashpoint in the debate over US immigration policy, which has been intensified by the recent deaths of two migrant children in US custody and a partial US government shutdown over President Donald Trump's $5 billion demand to lawmakers for a wall along the border with Mexico.

A previous incident in November when US agents fired gas into Mexico to disperse migrants triggered a call from Mexico for an investigation. A spokesman for Mexico's foreign ministry did not immediately respond to a request for comment.

Late on Monday, more than 150 Central American migrants approached an area of the border in Tijuana in the Playas neighborhood near the beach. Migrants said they thought security measures might be relaxed due to the New Year's holiday.

After midnight, US security personnel fired tear gas into Mexico as some migrants prepared to climb a border fence, according to the Reuters witness. During a second attempt, migrants began to pass youths and children over the razor wire along the fencing to the US side.

US Customs and Border Protection (CBP) said in a statement that the gas was aimed upwind of people throwing rocks on the Mexican side who obstructed agents from helping the children being passed over razor wire.

The CBP statement said agents had not directly targeted the migrants attempting to cross the fence with gas and pepper spray.

A Reuters witness did not see any migrants throwing rocks at US agents, and documented in one photo where a migrant had been hit by what appeared to be a gas canister.

CBP said most of the migrants attempting to cross returned back to Mexico while 25 people, including two teenagers, were detained.

Thousands of Central American migrants have been camping at shelters in Tijuana since arriving in November after traveling in caravans across Mexico to reach the US border, where many have hoped to request asylum.

Meanwhile, Democratic and Republican congressional leaders were expected to attend a briefing on border security at the White House as the government remains partially shut down and Trump asks in a tweet, "Let's make a deal?"

House Democratic leader Nancy Pelosi, who is expected to become speaker on Thursday, and Senate Minority Leader Chuck Schumer planned to attend. Pelosi said on Tuesday that Democrats would take action to "end the Trump Shutdown" by passing legislation on Thursday to reopen government.

Responding to the Democratic plan, however, White House press secretary Sarah Huckabee Sanders late on Tuesday night called it a "non-starter" and said it won't reopen the government "because it fails to secure the border and puts the needs of other countries above the needs of our own citizens".

Reuters/AP