Former NATO naval commander James Stavridis has warned that if Iran and Israel confront each other, oil supplies from the Middle East will be cut off.

"The escalation of tensions in the region could stop oil supplies from the Middle East," Stavridis told reporters.

"We are going through a dangerous moment, like the days before the outbreak of World War II, in addition to growing strategic tensions with China in the Pacific," he said.

"This great turmoil in the Middle East between Israel and Iran could develop into the worst-case scenario, and when all the tensions in the world happen together, it brings an unpleasant moment, like the late 1930s," he said.

"The next US president will help prevent conflict and tensions and bring about some kind of peace in the Middle East," Stavridis said.

US Vice President Kamala Harris has criticized Republican presidential candidate Donald Trump for not announcing the results of his long-term health tests.

"Trump's team doesn't want Americans to see who he is," Harris told a news conference at Andrews Air Force Base near Washington.

The Democratic nominee declined to comment on Trump's health, but said: "In his campaign statements, Trump talks about an unknown number and makes things up.

Asked if he believed Trump's mental capacity had declined, Harris said: "I suggest people watch the meetings he attends and decide for themselves.

On Saturday, a month before the election, Harris, 59, released information about his health. His doctor, Joshua Simmons, said Harris was in good health and could serve as president if he won the presidential race.

The Chinese military has launched an intensive exercise around the island of Taiwan with aircraft and warships to send a strong message to the island's separatists.

The Chinese military exercise has confused the Taiwanese government, which fears it could attack and take control of the island by force. Taiwan also called the exercise "irrational and warmongering" and said it had the capacity and strength to respond to any Chinese action and sent troops to the border.

"The Taipei Islands are on full alert and aircraft and warships are acting in accordance with the laws of war and have been ordered to respond to Chinese forces," the Defense Ministry said.

The United States also expressed concern about the destabilization of the situation in the region, calling the Chinese exercise unnecessary, saying: "There is a risk of further tensions in the region and called on Beijing to recognize itself.

For its part, the Chinese government said: "Their exercises are a strong warning to separatists who claim Taiwan's secession and independence.