NAIROBI, Kenya — A pirate fired a rocket-propelled grenade at a U.S. Navy destroyer shadowing a hijacked yacht with four Americans aboard Tuesday. Then gunfire erupted, the military said. U.S. special forces rushed to the yacht only to find the four Americans fatally wounded.

The experienced yacht enthusiasts from California and Washington are the first Americans killed by Somali pirates since the start of attacks off East Africa several years ago. One of the American couples on board had been sailing around the world since 2004 handing out Bibles.

Their deaths appeared to underscore an increasingly brutal and aggressive shift by pirates in their treatment of hostages.

Killing hostages “has now become part of our rules,” said a pirate who identified himself as Muse Abdi and referred to last week's sentencing of a pirate to 33 years in prison for the 2009 attack on the U.S. cargo vessel the Maersk Alabama.

“From now on, anyone who tries to rescue the hostages in our hands will only collect dead bodies,” he said.

ATHENS, Greece — Young demonstrators hurled rocks and fire bombs at riot police as clashes broke out today in Athens during a mass rally against austerity measures, part of a general strike that crippled services and public transportation around the country.Police fired tear gas and flash grenades at protesters, blanketing parts of the city center in choking smoke. Thousands of peaceful demonstrators ran to side streets to take cover. A police officer was attacked and his uniform caught fire in the city's main Syntagma Square, and his motorcycle was burned.At least two people were injured and another three arrested. One group of rioting youths smashed paving stones in front of the central Bank of Greece, but there were no immediate reports of serious damage.More than 30,000 protesters attended the Athens rally, which had been calm before the clashes. Protesters chanting “Don't obey the rich — Fight back!” marched to parliament as the city center was heavily policed.

SANAA, Yemen — Government supporters and police in Yemen have shot and killed at least one demonstrator while trying to dismantle a protest camp in the capital.That brings the death toll in nearly two weeks of protests to at least 12. The demonstrators are demanding an end to President Ali Abdullah Saleh's 32 years in power and complaining of corruption and poverty.Medics say Tuesday night's attack on the protest camp near Sanaa University also wounded 11 people.Yemen's Organization for Human Rights gave different casualty figures, saying two protesters were killed and 18 wounded.