Spokesman Robert Wood called the killing of the "Sunday Leader" newspaper editor, Lasantha Wickramatunga, and the attack on a TV station "deplorable acts," aimed at silencing independent voices in Sri Lanka.
Sri Lanka's president, Mahinda Rajapaksa, condemned the murder and ordered a full investigation.
However, Reporters Without Borders said the president and state media are partly to blame for the editor's murder. The Paris-based press rights group said "they had incited hatred against him and allowed an outrageous level of impunity to develop as regards violence against the press."
Lasantha Wickramatunga was fatally shot Thursday morning as he drove to work.
Human rights group Amnesty International says at least 14 media workers have been killed in Sri Lanka since the beginning of 2006, while others have been detained and tortured.
In other news, Sri Lanka's military is projecting that it will take control of the last rebel-held territory within weeks.
After capturing the rebel headquarters, Kilinochchi, last week, the military said Thursday it has overrun another rebel-held town of Pallai. It said the rebels appear to be preparing for a last stand against the government in the northern town of Mullaittivu, one of the Tigers' last remaining strongholds.
The Tamil Tigers have dismissed the army's recent gains as "insignificant" and have vowed to continue to fight back.
Tamil rebels have been fighting since 1983 to establish an independent homeland for Sri Lanka's minority Tamil community. More than 70,000 people have been killed in the violence.
Some information for this report was provided by AFP and Reuters.