U.S. stocks extended losses on Tuesday, as Wall Street became cautious ahead of Former FBI Director James Comey' s testimony.

The Dow Jones Industrial Average was down 47.81 points, or 0.23 percent, to 21,136.23. The S&P 500 lost 6.77 points, or 0.28 percent, to 2,429.33. The Nasdaq Composite Index shed 20.63 points, or 0.33 percent, to 6,275.06.

Comey is slated to testify Thursday in an open hearing on Capitol Hill, the first time since he was suddenly fired by U.S. President Donald Trump in early May.

It's highly expected that Comey will testify whether Trump asked him to shut down the FBI probe into former national security adviser Michael Flynn.

However, Comey will almost certainly not be able to discuss details of the ongoing FBI probe, which is now overseen by a special counsel, according to local media reports.

Overseas, investors prepared for a general election in the United Kingdom set for Thursday.

Meanwhile, geopolitical tensions in the Middle East also weighed on investors' sentiment.

The United Arab Emirates (UAE), along with Saudi Arabia, Bahrain and Egypt earlier this week cut diplomatic ties with Qatar, accusing the Gulf state of supporting and financing "terrorism" as well as interfering in their internal affairs.