Russia warned on Wednesday that "terrorist organizations" may arise in Syria should President Bashar al-Assad's regime fall under pressure from ongoing street protests.

"If the Syrian government is unable to hold on to power, there is a high probability that radicals and representatives of terrorist organizations will become entrenched," Interfax quoted a top foreign ministry official as saying.

Syrian security forces on Wednesday launched a massive military operation in the northwest of the country, activists said, while calling for demonstrations Thursday to mark six months of revolt against President Bashar al-Assad's regime.

"Six months. More than ever determined to (continue) the March 15 uprising," the activists wrote on their Facebook page, "Syrian Revolution 2011", despite the crackdown on protests which according to the United Nations has left 2,600 people dead, mostly civilians.

Hardline Israeli Foreign Minister Avigdor Lieberman warned Wednesday there will be "harsh and grave consequences" if the Palestinians persist with their plan to seek U.N. membership as a state.

Speaking shortly before a scheduled meeting with EU foreign policy Chief Catherine Ashton, Lieberman did not elaborate on the threatened consequences.

Nine people were killed in two days of clashes between tribal fighters who are opposed to Yemeni President Ali Abdullah Saleh and an elite military unit loyal to him, a tribal official said Wednesday.

The official, speaking on condition of anonymity, said violent clashes were ongoing in the town of Arhab, 40 kilometers north of the capital Sanaa, between Republican Guard troops commanded by Saleh's son Ahmed, and fighters from the Bakil tribe, the most influential tribe in the country.

Israel's public diplomacy minister said on Wednesday that he hoped "common sense would prevail" in former friend and ally Turkey in the diplomatic crisis the two governments are now embroiled in.

"Despite the attempts on the Turkish side to provoke an escalation, we are acting with restraint," Yuli Edelstein told public radio. "We are not pouring oil on the fire, in the hope that common sense will prevail."

A spate of attacks appearing to target security forces in central and west Iraq on Wednesday killed at least 17 people, including five policemen and two soldiers, officials said.

The violence, which also left around 50 wounded, comes with just months to go before U.S. forces are set to withdraw from the country completely, with questions over the capabilities of their domestic counterparts.

Iran's elite Revolutionary Guards have warned members of an Iraq-based Kurdish separatist group they will be fought "to the end" unless they lay down their arms or leave, local media reported Wednesday.

"Our message is that Islamic Republic of Iran will not tolerate any armed terrorist group which is the product of Americans. They will be confronted decisively till the end," the deputy commander of the force, Brigadier General Hossein Salami said, referring to the Party of Free Life of Kurdistan (PJAK).

The U.S. and French ambassadors traveled to the Damascus suburb of Daraya on Tuesday to attend a condolence ceremony for slain Syrian activist Ghiyath Matar, who reportedly died under torture, activists said.

"The ambassador of the United States arrived in Daraya along with the French ambassador to offer condolences after the death of Ghiyath Matar," the rights activists said on Twitter.

Qatari Prime Minister Sheikh Hamad bin Jassem al-Thani said on Tuesday that Syria must stop its "killing machine" and pull its army out of cities, in remarks at the end of an Arab League meeting.

"We cannot accept this killing machine. We cannot allow people to be killed this way," he told a news conference at the end of a regular meeting in Cairo of the pan-Arab body, of which Syria is a member.

Egypt's ex-intelligence chief Omar Suleiman testified on Tuesday in the trial of former president Hosni Mubarak who faces charges of involvement in the killings of protesters, state television said.

There were no further details about the testimony which was held behind closed doors.