Image copyright Getty Images

Secretary of Homeland Security Kirstjen Nielsen is testifying to Congress and being grilled on the deaths of migrant children and other border issues.

Democrats on the panel have accused her of lying or being unaware of the situation at the US-Mexico border.

The hearing comes after the death of two children in US custody last year.

Mrs Nielsen is the highest-ranking Trump official to be questioned by Congress since Democrats took control of the House of Representatives.

The hearing before the Democrat-controlled House Homeland Security Committee began on Wednesday with Mrs Nielsen saying that "vulnerable populations - especially children - are coming into Department of Homeland Security (DHS) custody sicker than ever before".

What did she say about child deaths?

When asked about the deaths of two young migrant children in DHS custody last December, Mrs Nielsen said that her department is awaiting a medical examiner's report before releasing their own findings.

But she told Democrats that she had no idea when the medical examiner's report will conclude, and said they are currently investigating "extenuating circumstances" including the condition the children were in during their journey to the US.

When asked what policies have recently been implemented to assist young migrants, Mrs Nielsen said that all girls over 10 are now receiving pregnancy tests by US immigration agents, due to the high rate of sexual abuse during the passage.

Democrats reacted angrily when she said she did not know how many migrant children are currently in US detention.

Media playback is unsupported on your device Media caption US child migrants: Five things to know

Last Friday, one immigration detention facility in Texas said that there were 16 infants under one year old in their care.

By Tuesday, Immigration and Customs Enforcement said that all but one of the babies at the South Texas Family Residential Center had been released.

According to immigration advocacy groups, more vulnerable people - such as pregnant women, young children and the elderly - are now being detained after crossing the border.

These people, advocates say, had previously been released after agents determined they were not a threat to communities in the US.

What did lawmakers say?

California Congresswoman Nanette Barragán told Mrs Nielsen bluntly "either you are lying to this committee, or you don't know what's happening at the border".

Democrats pressed her on how many children have been put in "cages", which Mrs Nielsen said have not been used at DHS.

"I've seen the cages. I just want you to admit that cages exist," said Chairman Bennie Thompson.

"Sir they're not cages. They are areas of the border facility that are carved out for the safety and protection of those who remain there while they are being processed," she said in a tense exchange.

Texas Democrat Al Green told her "white babies would not be treated the way these babies of colour are being treated".

Image copyright Getty Images Image caption Mississippi Democrat Bennie Thompson is chairing the hearing

Republican Congressman Clay Higgins, said that the number of migrants that came to the US-Mexico border last month - 75,000 people - is the same as how many US soldiers landed on French beaches to liberate the country from Nazi Germany.

He called the situation "a D-Day every month".

What else did Nielsen say?

She also said that there have been more apprehensions of illegal migrants in the first six months of this year than all of last year put together.

One million people are currently expected to arrive at the US southern border this year, she said.

Media playback is unsupported on your device Media caption The sound of migrant children separated from parents

Mrs Nielsen repeatedly referred to the situation at the US border as a "crisis", and "emergency", and a "direct national security threat" during her testimony.

She also said that a deal is being reached with Central American countries to prevent people from smuggling and forming caravans to travel to the US in groups.

The homeland security chief said she was aware of parents being deported back to their home country without their children, but said that those parents had been given the opportunity to take their kids with them but chose not to.