The United States remains committed to the security of its "important NATO partner" Bulgaria, U.S. Secretary of State Hillary Clinton says.

Clinton made the comments Sunday during an official visit to the Eastern European nation, in which she reiterated U.S. support for its security at a news conference in Sofia with Bulgarian Prime Minister Boyko Borisov.

"Let me say how committed the United States is to Bulgaria's security," Clinton said. "We are NATO allies. We take very seriously our Article 5 obligation for collective defense. Bulgaria has been an important, productive partner of NATO."

Thanking Bulgaria for its contribution of troops with the NATO-led forces battling Taliban extremists in Afghanistan, Clinton said the security cooperation between Washington and Sofia will only deepen over time.

She cited NATO efforts to develop a missile defense system in Europe as an example.

"With respect to security cooperation going forward, we want to make sure that we consult closely with our Bulgarian friends about how the United States and Bulgaria bilaterally and through NATO will make sure that Europe has the best defense in terms of missile defense and other capabilities in order to protect Bulgaria and all of our European allies," Clinton said.

Bulgaria's leaders had offered last year to host an advanced radar station as part of a U.S./NATO missile shield radar if Turkey refused to do so. However, Ankara later agreed to accept the installation.

The United States says the shield is necessary to protect the continent from the proliferation of ballistic missiles in rogue states such as Iran and North Korea but Russia opposes it without binding guarantees it won't be used against its own strategic rocket forces.

Russian Foreign Minister Sergei Lavrov said Saturday no breakthroughs on the missile shield disagreements had been reached during a global security conference last week in Munich, Germany.

Clinton said in Sofia that Washington is seeking to expand military cooperation with Bulgaria, adding the country "has proven to be a very capable partner for whom we have the greatest respect and to whom we owe our NATO responsibility of providing defense."

The secretary of state also cited energy security as a key issue for Bulgaria, declaring Washington "will stand with the Bulgarian people and government as they work to be able to provide affordable energy that meets your needs."

The statement was interpreted as a reference to the potential for shale gas development in Bulgaria, which is seen by backers a vital way to lessen dependence on Russia for natural gas supplies.

The Bulgarian Parliament last month imposed a moratorium on shale gas exploration after a wave of citizen protests against the issuance of a drilling permit for U.S. energy giant Chevron.

Environmentalists contend the drilling process, known as hydraulic fracturing, is dangerous but a coalition of rightist Bulgarian political parties has demanded the ban be lifted, the Sofia News Agency reported.

Clinton said she would send Special U.S. Envoy for Eurasian Energy Richard Morningstar to Bulgaria this week "to have expert conversations about how we can be more helpful in protecting your environment and advancing your energy security goals."