WASHINGTON (AFP) - US Secretary of State John Kerry choked back tears on Wednesday, saying he had been "humbled" by the outpouring of support for his wife, who remains in hospital for tests after an apparent seizure.

"Teresa is doing better, under evaluation and we hope improving," Mr Kerry said, opening two days of talks with senior US and China leaders.

Mrs Teresa Heinz Kerry, 74, has been in hospital since displaying seizure-like symptoms on Sunday while the couple were vacationing with their family at their home on Nantucket Island, off Cape Cod, Massachusetts.

Mr Kerry had flown on Tuesday from the Boston hospital where his wife is undergoing tests to host a dinner with senior Chinese leaders, including Vice Premer Wang Yang and State Councilor Yang Jeichi.

He thanked friends and colleagues for the "remarkable outpouring of good wishes," adding "we are very, very humbled by the expressions of support".

"I think she's coming along and I know when she's able to she'll thank everybody herself," said Mr Kerry, his voice breaking with emotion as he paused to compose himself.

Mr Kerry, 69, the 2004 Democratic presidential candidate, had been celebrating the July 4 holiday weekend with his family on his first long break since becoming secretary of state in February, when his wife fell ill.

After being at his wife's bedside since she was rushed first by ambulance to a local hospital on Nantucket, Mr Kerry travelled back to Washington for the long-planned talks with Chinese officials.

But State Department officials have said that if he needs to return abruptly to Boston, Deputy Secretary Bill Burns will then represent him at the talks. Meanwhile, family members remain with Mrs Kerry at the hospital.

Heinz Kerry, a former interpreter at the United Nations, and Kerry have been married for 18 years. They have 5 children between them.

Mrs Heinz Kerry was previously married to senator Henry John Heinz, the heir to the Heinz ketchup fortune, who was killed in a helicopter crash in 1991.

Doctors have so far ruled out stroke, heart attack or brain tumour as the cause of her illness.

A committed environmentalist like Mr Kerry, they first met at an Earth Day summit in 1990. They then met again two years later in Rio for an Earth Day event, after Mr Heinz's death.