U.S. President Donald Trump has been briefed about the London terror attack and called it "big news."

"I was just getting an update on London. Some big news having to do with London," Trump said, as he entered the Roosevelt Room in the White House for a meeting on women's health care.

Separately, the State Department said it was closely monitoring the "concerning situation" and was in particular trying to find out whether any U.S. visitors had been caught up in it.

"Our embassy in London is monitoring the situation closely, " acting spokesman Mark Toner said.

"We stand ready to provide all possible consular assistance should we become aware of any affected U.S. citizens.

"We urge U.S. citizens in London to contact family members and loved ones to notify them that you are safe, to avoid the area and monitor local news."