BEIRUT, Lebanon - Five pro-Syrian Lebanese officials arrested as suspects in the probe into the murder of ex-premier Rafiq Hariri six months ago are due in court on Wednesday after being grilled by UN investigators.

The five include three former security chiefs detained in dawn sweeps on Tuesday, the head of President Emile Lahoud's presidential guard and a former MP who both turned themselves in after arrest warrants were issued.

It is the first major development in the probe into Hariri's killing in a Beirut bomb blast in February that has been widely blamed on former power broker Syria and its political allies in government in Lebanon at the time.

The United States described the arrests as a "very dramatic development," noting the officials' long-standing ties to Syria, and reiterated its complaint over the lack of cooperation from Damascus.

The arrests have also ratcheted up the pressure on Lahoud, the Damascus protege who himself has long denied allegations of complicity in the assassination and resisted pressure to stand down.

His presidential guard chief General Mustafa Hamdan, who was previously questioned in June and his house and office searched, turned himself in to UN investigators after Lebanon issued a warrant for his arrest.

Former general security chief, General Jamil al-Sayed, ex internal security head General Ali al-Hage and former army intelligence director General Raymond Azar were taken to the UN commission of inquiry's headquarters for questioning after their arrests in dawn raids.

Former pro-Damascus minister Nasser Qandil, who was visiting the Syrian capital, was also summoned to appear before the inquiry commission.

German prosecutor Detlev Mehlis, who heads the UN commission, is due to hold a press conference on Thursday.

According to Future TV, owned by the family of Hariri, the arrests followed the discovery last month of arms caches in Lebanon.

"This is the start of the process of uncovering the truth, and other arrests will follow," MP Saad Hariri, the slain prime minister's son, told Radio-Orient from Paris.

Hariri's killing triggered massive opposition protests and heightened international pressure on Syria, which pulled its troops out of the country in April after a three-decade military presence.

Legislative elections were held in May and June which for the first time saw anti-Syrian politicians dominate parliament.

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