Three International Committee of the Red Cross employees kidnapped by Yemeni tribesmen are in good health, but the demands of their captors remain unknown, an ICRC spokeswoman said on Tuesday.

But a tribal dignitary who is involved in negotiations to secure the hostages' release told Agence France Presse later on Tuesday that the kidnappers were demanding that the trial of fellow tribesman Abdelbaset al-Markashi be moved to their hometown Jaar from the main southern city of Aden.

The vacant Syrian seat at the Arab League has not been given to the opposition, even though its leader addressed the bloc's last summit, the League's secretary general said on Tuesday.

"The opinion was that if they (opposition) form a government, which they have not done yet, then they can become a representative," Nabil al-Arabi told reporters in Dubai.

Russian President Vladimir Putin on Tuesday warned against any moves that would further destabilize the situation in Syria, speaking after talks with visiting Israeli Prime Minister Benjamin Netanyahu.

"In this crucial period it is especially important to avoid any moves that can shake the situation," Putin was quoted as saying by news agencies, days after Israeli forces launched air strikes against regime targets in Syria.

Jordan's foreign ministry said on Tuesday Amman will host a meeting of the so-called "Friends of Syria" group next week to discuss latest developments in Syria's civil war.

"The foreign ministers of Jordan, Saudi Arabia, the United Arab Emirates, Egypt, the United States, Britain, France, Turkey, Germany and Italy will hold a meeting in Amman in mid-next week," ministry spokeswoman Sabah Rafei told Agence France Presse.

A Moroccan street vendor in the southern city of Marrakesh died in hospital after setting himself on fire in protest at having his goods confiscated by the authorities, various sources said on Tuesday.

Mbarek el-Karassi, 32, died on Monday evening from severe burns to the face, hands and stomach, said Mohammed Ghalloussi, a local representative of the Moroccan Association for Human Rights.

Turkish Prime Minister Recep Tayyip Erdogan on Tuesday firmly rejected a Syrian proposal to hold a joint probe over a twin car bomb attack that killed 51 people.

"The administration in power in Damascus is illegal ... how can we recognize a structure that isn't even recognized by its own citizens," Erdogan told reporters before leaving for a meeting with U.S. President Barack Obama in Washington on Thursday.

Syria's opposition will receive additional assistance if President Bashar Assad refuses to attend a peace conference proposed by the U.S. and Russia, U.S. Secretary of State John Kerry said Tuesday.

"If he miscalculates about that as he miscalculated about his own country's future over the course of the last years, it is clear the opposition will be receiving additional support," Kerry said during a visit to Stockholm.

A Saudi convicted of murder was beheaded on Tuesday, the interior ministry announced, raising to 40 the number of people executed in the conservative Muslim kingdom this year.

Maneh al-Daen was found guilty of stabbing to death a fellow tribesman, Nasher al-Daen, the ministry said in a statement carried by the official SPA news agency.

Kuwait's government boycotted a parliament session on Tuesday after MPs filed requests to grill the oil and interior ministers over alleged violations, including a $2.2 billion penalty payment to U.S. Dow Chemical.

"Due to the absence of cabinet ministers and as I was informed the government will not attend, I adjourn the parliament session until Wednesday," speaker Ali al-Rashed told those MPs present in the chamber.

A campaign calling for the ouster of Egyptian President Mohamed Morsi and for early presidential elections has gathered over two million signatures, organizers told Agence France Presse.

The "Tamarod" (Rebellion in Arabic) campaign seeks to withdraw confidence from the Islamist leader because he has failed the revolution that brought him to power, they said.