US Secretary of State Hillary Clinton will travel next week to a security conference in Germany before visiting Bulgaria to discuss their joint work in Afghanistan, a spokeswoman said Friday.

The chief US diplomat will take part in the annual Munich Security Conference on February 3-5, which "brings together global leaders to discuss common security challenges," Clinton spokeswoman Victoria Nuland told reporters.

Clinton will also use the occasion to have "bilateral meetings with a number of her European and other counterparts from around the world," Nuland said without giving details of the agenda for talks.

When she attended last year's conference, Clinton advocated support for open and accountable governments across the Middle East -- shaken by mass protests across the region -- despite the short-term risks of chaos and instability.

The new START nuclear arms reduction treaty also officially came into force in Munich when Clinton and Russian counterpart Sergei Lavrov exchanged ratification documents at the security conference.

On February 5, Clinton will travel to Sofia, Bulgaria to meet with senior Bulgarian officials to discuss the democratic transitions in the Middle East, joint support for Afghanistan, and energy security, Nuland said.

Bulgaria has 614 soldiers, army instructors and military doctors in the war-torn country.

Clinton and her Bulgarian hosts will also discuss bilateral cooperation in international law enforcement, Nuland said.