Iran is threatening to put the 15 British marines and sailors it captured on trial for violating its territorial waters.
"This is a very serious situation," declared British Prime Minister Tony Blair, who spoke out on the crisis for the first time. "They should not be under any doubt at all about how seriously we regard this act, which was unjustified and wrong."
For More on This Story Watch "World News With Charles Gibson" at 6:30 p.m.
Traveling in the Middle East, U.S. Secretary of State Condoleezza Rice said, "They ought to be released … and released immediately."
The incident only adds to the tensions with Iran over its nuclear program. On Saturday, the United Nations Security Council imposed new sanctions. Iran accused the U.N. of "illegal bullying" and threatened to retaliate by cutting back the access of U.N. inspectors.
"This is an international concern -- it's not between the U.S. and Iran alone," said Judith Kipper, a Middle East expert with the Council on Foreign Relations. "So far, they don't seem to fully comprehend that that isn't the case and they may go to the brink -- which is very, very dangerous for that area and the entire globe."
Others say sanctions alone are not enough.
"I think we need much more increased pressure," said John Bolton, former U.S. ambassador to the United Nations. "Frankly, the only real way to be sure Iran doesn't get nuclear weapons is to change the regime in Tehran."
Nobody knows what Iran's intentions are in holding the British sailors and marines. The big worry is that they will link their fate to resolving the nuclear issue and somehow turn them into a bargaining chip.
"If the 15 marines, in fact, become hostages because of U.N. sanctions, if they are linked, obviously the military option is on the table," said Kipper. "But I do not think at this time that a military option can resolve any of the problems that the international community has with Iran."