The wife of US Secretary of State John Kerry remained in hospital Monday for further tests after suffering what may have been a seizure, a source close to the family said.

Teresa Heinz Kerry, 74, spent the night in Massachusetts General Hospital in Boston where she was flown late Sunday after being rushed to a local hospital on Nantucket island.

The family had become concerned on Sunday "when they noticed symptoms which were consistent with a sort of seizure," the source told AFP.

"She is still being evaluated, and there is no diagnosis of any kind yet," he added.

Kerry remained by his wife's side, and no decision had yet been made when the top US diplomat would resume his official duties.

Heinz Kerry, a multilingual philanthropist born to Portuguese parents in colonial-era Mozambique, had arrived at the Nantucket hospital in a critical but stable condition. She was transferred to Boston after being stabilized.

Kerry, the 2004 Democratic Party presidential candidate, had been celebrating the July 4 national holiday weekend with his family at their home on Nantucket Island, an upscale tourist hotspot off Cape Cod, Massachusetts, on his first long break since becoming secretary of state in February.

The couple have been married for 18 years, and it is a second marriage for both of them. She has three sons and Kerry has two daughters.

Kerry, 69, has kept up a punishing schedule since succeeding Hillary Clinton as secretary of state and had been expected to return to the Middle East soon as he tries to relaunch Israeli-Palestinian peace talks.

This week he is due to host a two-day strategic dialogue with top Chinese leaders, which is set to begin with a dinner Tuesday evening.

No decision had yet been made on whether he will be able to attend, the source said.

Kerry had headed straight for Massachusetts to spend time with his family after returning home on July 3 from a 12-day tour to several Middle Eastern nations, as well as India and Brunei.

He has already visited 27 countries, spent some 68 days out of five months on the road, traveled some 134,691 miles and spent a total of 293 hours, or some 12 days, in his Air Force plane.

A former interpreter at the United Nations, Heinz Kerry was married to senator Henry John Heinz, the heir to the Heinz ketchup fortune, who was killed in a helicopter crash in 1991.

She inherited many of his trusts and is very wealthy.

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

In 2009 Heinz Kerry revealed she had been treated for breast cancer.

Kerry's spokesman Glen Johnson said in a statement Sunday that "the family is grateful for the outpouring of support it has received" but asked for "privacy at this time."