HAVANA: A 61-year-old US swimmer battled shoulder pain and asthma in shark-infested waters off Cuba on Monday at the start of a trip to the United States aimed at promoting closer ties between the Cold War foes.

"We want these three days to help the two countries get closer," Diana Nyad told a press conference in Havana before embarking on her marathon swim, adding she also hoped to show that people can lead active lives after 60.

"I am a better athlete today than I was at 29," she told CNN.

A CNN producer following the journey by boat said that after 18 hours, Nyad had completed about a quarter of the 100-mile (166-kilometer) trip from Havana to the Florida Keys and was battling more than currents and the risk of sharks.

"Everything's going well. She's had a little bit of a problem with her shoulder, it's been bothering her," said the producer, Matt Sloane, adding that Nyad had been treated with ice and anti-inflammatories.

She was also having "a little bit of trouble breathing" and had used an inhaler, he said.

"Otherwise, she's really strong," the producer added.

Nyad tried to cross the Florida Straits in 1978 but was forced to abandon the bid after 42 hours due to poor weather.

The following year, she set an open sea record by swimming from the Bahamas to the Florida Keys -- a journey that is the same distance as her current swim, but a feat she described as much less dangerous.

Five yachts and four kayaks are accompanying her on the journey, along with a 45-person support team, including trained shark divers and electronic devices designed to repel the predators.

Australian swimmer Susie Maroney, at 22, became the first person to swim from Cuba to the United States in 1997, though she used a shark cage.

Nyad told NBC television's "Today" show that she declined to use a cage, adding: "I don't want to have that asterisk next to my name."

She will stop and tread water every 45 minutes to rehydrate and every 90 minutes to eat, organizers said.

The distance swimmer, who turns 62 later this month, set a record for circling the island of Manhattan at the age of 50, clocking in at seven hours and 57 minutes.

"It doesn't have to be a big moment," she told NBC about her latest feat. "It just means be engaged in your life. Don't let it go by. We all have one life and it's a one-way street."

The United States has not had diplomatic relations with Cuba since 1961, when it slapped a trade embargo on the communist-ruled island. AGENCIES