BUENOS AIRES, Argentina — President Trump has a busy schedule with world leaders as he arrives in Argentina for the G-20 summit. But one planned meeting will not happen after all.

Shortly after Air Force One left Washington for Argentina, President Trump canceled — via airborne tweets — a high-profile meeting with Russian President Vladimir Putin. He did so "based on the fact that the ships and sailors have not been returned to Ukraine from Russia."

Russia continues to hold three Ukrainian naval vessels and more than 20 sailors they seized last weekend in the Black Sea. But a short time earlier on the south lawn of the White House, the president had said the Putin meetings were still on.

"We haven't terminated the meeting. I think it's a very good time to have the meeting," he said.

The abrupt reversal occurred after the president conferred with top national security officials. It also came shortly after word of Michael Cohen's plea deal, in which he admitted to lying about a Trump organization development project in Moscow — a topic that surely would have prompted intrusive questions at the G-20.

Russia's recent naval aggression will test how tough Mr. Trump intends be to protect Ukraine, which he accused former President Barack Obama of failing to do. The Trump administration has sent lethal weapons like anti-tank missiles to Ukraine and U.S. National Guard forces have trained the Ukrainian army fighting Russian-backed separatists.

Another big G-20 topic is escalating trade tensions with China. The president said China wants an agreement to avert hundreds of billions in new U.S. tariffs that could take effect in January.

"I don't know if I want to do it because what we have right now is billions and billions of dollars coming into the United States in the form of tariffs or taxes, so I really don't know," he said.