President Obama says he fears battle-hardened terrorists from Syria will stage attacks in the United States or Europe. REUTERS/Yuri Gripas

U.S. President Barack Obama and Rep. Peter King (R-N.Y.) Sunday expressed fears terrorists being trained in Syria will return home to the United States or Europe to conduct attacks.

In an interview aired Sunday on ABC's "This Week," Obama said terrorists are gaining strength, noting some Europeans sympathetic to terrorist causes have traveled to Syria, "and now may travel into Iraq, getting battle-hardened. Then they come back, they have European passports, they don't need a visa to get into the United States. We’re spending a lot of time, and we have been for years, making sure that we are improving intelligence so we can respond to that.”

Obama also said U.S. intelligence capabilities need to be improved.

King, a member of the House Homeland Security Committee, said the situation in Syria presents the most serious threat to the United States, noting there are at least 100 Americans fighting with the rebels now.

"So any of these people can go back in the United States and they could carry out that type of attack that they’re being trained in in Syria," he said on "This Week." "And the terrorists in Syria are extremely sophisticated, very advanced. And so thousands of people can go back to Europe and come here. That’s over 100 Americans who could come back to it. All we have to do is miss one or two of them, and we could have a very, very lethal attack here in the U.S."

In response to an ABC report Syrian terrorists are developing a new bomb to be smuggled aboard civilian aircraft, King said it is imperative airport security be increased overseas.

"But basically, we are saying anyone that (inaudible) to fly to the U.S., they have to increase their security. We’re going to be pushing it. I can’t go into all the details, but overseas airport security is a real concern we’ve had in the U.S.," he said.