SANAA, Sept 18 (Reuters) - Armed Shi'ite rebels pushed into Yemen's capital Sanaa after clashing with the army in the city's northwest outskirts on Thursday, security sources and residents said, in an escalation of weeks of fighting and protests.

Residents of northwest al-Shamlan district told Reuters the Shi'ite Houthi gunmen were now advancing along Thalatheen Street, a major route into the western edge of the city.

The fighting has further destabilised an impoverished country also struggling to overcome a secessionist movement in its south, the spread of an al Qaeda insurgency and other threats.

The stability of Yemen is a priority for the United States and its Gulf Arab allies because of its strategic position next to top oil exporter Saudi Arabia and shipping lanes which run through the Gulf of Aden.

A military source said Houthi gunmen had also attacked an army camp on the southern entrance of the capital, but soldiers repelled the assault.

The Houthis, who belong to the Zaydi sect of Shi'ite Islam, have been involved in a decade-long conflict with the Sunni-dominated government, fighting for more control and territory in the north.

In recent weeks, Houthi protesters have been blocking the main road to Sanaa's airport and holding sit-ins at ministries calling for the ousting of the government and the restoration of subsidies cut by the state in July as part of economic reforms.

At least 42 people have also been killed in clashes involving Houthi fighters in different parts of the country since Tuesday.

Critics say the Houthis are trying to grab power and carve out a semi-independent state for themselves in the north - something they deny.

The United Nations' special envoy to Yemen meet Houthi leader Abdulmalek al-Houthi in Saada province on Wednesday to try and find a way out of the conflict. The three-hour meeting was "constructive and positive" Jamal Benomar was reported as saying.

The Houthis said on Monday they would no longer take part in negotiations with the Yemeni government about their grievances because of what they termed "foreign intervention" in the discussions. (Reporting by Mohammed Ghobari; Writing by Yara Bayoumy; Editing by Andrew Heavens)