US surveillance drone lost near Zliten, while rebel-held Misurata comes under rocket attack for the first time in weeks.
A NATO aircraft downed in Libya was a drone helicopter that had earlier lost radar contact with its operators, US defence officials said on Tuesday.
The MQ-8B Fire Scout, a small, pilotless helicopter used for surveillance, was one of two deployed to the Libyan conflict, the officials told the AFP news agency.
It was the first confirmation of the use of such aircraft in Libya.
It said the helicopter had been shot down by Libyan armed forces near Zliten, about 160km east of the capital, Tripoli. NATO denied losing any attack helicopters.
A US defence official told AFP that the cause of the break in radar contact between the Fire Scout and NATO's command center in Naples, Italy, was unknown.
It was unclear whether the small helicopter, equipped with cameras and sensors for surveillance flights off naval ships, had been shot down or suffered mechanical or communications problems.
Farther east of Zliten, in the rebel-held city of Misurata, rockets fired by forces loyal to Libyan leader Muammar Gaddafi hit the city centre for the first time in weeks, proving the city remains in range of his forces.
Four rockets hit the area on Tuesday, inspiring fear among residents that there may be a return to last month's violence. Rebel forces managed to push out Gaddafi's men in May after weeks of heavy fighting.
Drone helicopter was gathering intelligence
The Pentagon had previously announced the deployment of two armed Predator drones for the NATO-led air campaign against Libyan leader Muammar Gaddafi's forces, but had not previously cited any role for the unmanned helicopter.
Both Fire Scouts were deployed to the Mediterranean Sea aboard the USS Halyburton, a frigate.
The alliance lost track of the helicopter at 9:20am local time in Libya's central coastal area, NATO spokesman Mike Bracken said.
"This drone helicopter was performing intelligence surveillance and reconnaissance over Libya to monitor pro-Gaddafi forces threatening the civilian population," Bracken told a news conference in Brussels via videolink from Naples.
The Fire Scout, manufactured by Northrop Grumman, carried out its first flight in 2006 and was deployed on a guided-missile frigate, the USS McInerney, in 2009 for counter-drugs efforts.
The Fire Scout can reach an altitude of 6,000 metres, fly at a speed of more than 200kph and stay in the air for more than eight hours, employing sensors and radar to find and track targets.
The US navy had a fleet of 15 Fire Scouts before Tuesday's incident and plans to build 168 of the helicopters, proposing funding for three Fire Scouts in 2011 and a dozen in 2012.
Three Fire Scouts have also been deployed for operations in Afghanistan, officials said.
US tightens sanctions amid debate
The loss of the drone in Libya came as the US Treasury Department said it was blacklisting nine companies owned or controlled by the Libyan government.
The US has said it will consider lifting sanctions against officials who "distance" themselves from the government.
In Congress, politicians remained divided over whether Barack Obama, the US president, has overstepped his legal authority by backing the air campaign against Gaddafi's forces without seeking authorisation from politicians.
Since April, when NATO assumed command of the air campaign, US planes have carried out about 60 sorties on anti-aircraft targets in Libya, while Predator drones have fired missiles about 30 times, according to the New York Times.
On Tuesday, high-ranking senators John Kerry and John McCain, both former presidential candidates, jointly introduced a measure that would formally authorise US military action in Libya for up to one year.
Halting the US air campaign would "doom the Libyan operation" and "undermine the very core of NATO," said Kerry, a Democrat.
Representative Dennis Kucinich, another Democratic former presidential candidate, has proposed an amendment to the forthcoming overall defence spending bill that would cut off funding for the Libya operation.