The President said they included an American pilot of Latin American origin, arrested in the southwest border state of Táchira.
He said the pilot was found in possession of "all kinds of documents" and was being interrogated by the authorities, though he did not identify him. The Venezuelan government has made many similar claims in recent years, without ever substantiating them.
Maduro also announced Saturday a series of measures, including visa requirements for U.S. citizens and the downsizing of the U.S. Embassy in Caracas, to counteract what he called U.S. "interference" in his country.
Venezuela has in recent weeks accused the United States of being behind an alleged coup plot.
Speaking at an "anti-imperialist" rally in the capital, Maduro said visas would now be required for all U.S. visitors and that the U.S. Embassy in Caracas would now need foreign ministry approval for any meetings. The Embassy, which he said had more than 100 staff, is to be reduced to a number closer to the 17 Venezuelan diplomats based in Washington.
Moreover, a group of prominent U.S. officials, current and retired, will be banned from entering Venezuela because of what Maduro said was their involvement in "bombing Iraq, Syria and Vietnam" and other "terrorist" actions. The officials include George W. Bush, former U.S. Vice President Dick Cheney, former CIA Director George Tenet and several current members of Congress, including Ileana Ros-Lehtinen, Bob Menendez and Mario Diaz-Balart.
Following the Maduro's announcement Diaz-Balart reacted via Twitter, saying he has "always wanted to travel to a corrupt country that is not a free democracy. And now Castro's lap dog won't let me!"
The move comes after the U.S. government last month approved a law under which Venezuelan officials allegedly involved in human rights violations are to have their visas revoked and their U.S. assets frozen.
A relatively small, but noisy crowd, dressed mostly in revolutionary red, applauded and cheered the measures announced by the President from a platform outside the presidential palace in downtown Caracas.
Four missionaries from Bethel Evangelical Free Church in Devils Lake, North Dakota were released by Venezuelan authorities on Saturday, a church official said.
Pastor Bruce Dick said the missionaries arrived in Venezuela on February 20 and were detained a few days ago.
"We love the Venezuelan people and have served alongside them for over 12 years," Dick said. "We have been praying along with hundreds or thousands of others for their release and for those in Venezuela who also have been affected by this."
It is unclear if the detention and release of these Americans is connected to Maduro's charges of espionage.