He says Mr Wahbh was held for 53 days by the rebel Free Syrian Army.
There were also some skirmishes between security forces and rebel fighters from the Free Syrian Army.
Louay Almokdad, a spokesman for the rebel Free Syrian Army, said he was shocked by the report.
Meanwhile, the Free Syrian Army (FSA) - the main rebel fighting force - condemned the seizure of the UN observers.
Mika Yamamoto, 45, worked for the independent Japan Press news agency and had been reporting on the Free Syrian Army.
Earlier, the commander of the rebel Free Syrian Army said his group would work "under the umbrella" of any new government.
BBC: Syria opposition names Ghassan Hitto PM for rebel areas
Riad al-Asaad, who became head of the Free Syrian Army rebel coalition.
On 25 June, citizen journalist Ghias Khaled Al-Hmouria was shot dead in Douma while filming an operation of the rebel Free Syrian Army.
But Louay Almokdad, political and media coordinator for the Free Syrian Army, told CNN that the rebels lack access to chemical weapons and surface-to-surface missiles.
The rebel Free Syrian Army (FSA) is believed to be supported financially by Sunni-ruled Gulf states such as Saudi Arabia and Qatar, as well as Turkey.
White said the armed opposition has grown in power and includes growing numbers of defectors and civilians called the Free Syrian Army as well as jihadist groups.
The opposition meetings in Istanbul come two days after a top military defector said armed rebel groups have aligned under the leadership of the Free Syrian Army.
Reuters news agency quoted a spokesman for the main Syrian rebel group, the Free Syrian Army, as saying al-Nusra had overrun state security and military intelligence compounds in al-Shaddada.
The defections included a Syrian general and at least four other officers, according to a Turkish Foreign Ministry official and a commander in the Free Syrian Army camp in Turkey.
Posters of Bashar al-Assad, who succeeded his father Hafez in 2000, are everywhere on Jabal Muhsin, while rebel Free Syrian Army flags outnumber Lebanese ones in the Sunni areas nearby.
It includes allegations that rebel forces such as the Free Syrian Army recruited and used children, despite the FSA's stated policy of not recruiting anyone younger than 17 years of age.
The US might argue that its vetting to this end is working: non-Islamist units of the Free Syrian Army, a loose network of rebel groups, seem to be the major beneficiaries so far.
Salim Idriss, a general with the rebel Free Syrian Army, said Wednesday that his forces were outgunned by Hezbollah fighters and appealed to the international community for heavy weapons to help his fighters in Qusayr.
Riad al-Asaad, leader of a now-sidelined rebel umbrella group known as the Free Syrian Army, had his right foot amputated following the blast late on Sunday, according to an activist in the town of Mayadeen where the attack took place.
Across Syria a picture is emerging of government forces holding the centre of big towns and the main roads, and the Free Syrian Army becoming increasingly active and almost controlling parts of cities like Homs and areas of the countryside.
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