Syria has shown "flagrant disregard" for Lebanon's sovereignty and continues to arm the Lebanese Hezbollah guerrilla group, U.S. Ambassador to the United Nations Susan Rice said on Thursday.

"Syria especially has displayed flagrant disregard for the sovereignty, territorial integrity, unity and political independence of Lebanon," Rice told reporters as the UN Security Council held closed-door discussions on Lebanon.

"Syria continues to provide increasingly sophisticated weapons to Lebanese militias, including Hezbollah, despite (Security Council) resolution 1680, which calls on Syria to
undertake measures against the movement of arms into Lebanese territory," she said.

Hezbollah remained the "most significant and most heavily armed Lebanese militia," Rice said, adding that it could not have achieved this without Syrian aid in acquiring Syrian and Iranian weapons.

She said Damascus' decision to issue arrest warrants for 33 senior Lebanese officials and foreign nationals was another example of Syrian violation of Lebanese sovereignty and breach of Syrian pledges to respect its neighbor's independence.

Tension has escalated in Lebanon over a UN tribunal investigating the 2005 assassination of former Lebanese Prime Minister Rafik al-Hariri and the country almost sank into new turmoil after reports that the court will indict members of the Shi'ite militant group Hezbollah in the coming months.

Hezbollah, part of Lebanon's national unity government, has denounced the UN-backed court as a tool of U.S. and Israeli policy and called on Prime Minister Saad al-Hariri, Rafik's son, to repudiate the tribunal.

The court issued a statement on Wednesday condemning what it described as an attack on its staff in Beirut and said it would not be deterred in its inquiry.

Earlier this week, Syria's President Bashar Assad accused the United States of sowing chaos overseas, snubbing Washington's efforts to improve ties with Damascus and steer Syria away from Iran's influence.