By Kylie Morris BBC, Bangkok

The US-based group Human Rights Watch is the latest to call for police to step up their efforts to find the missing lawyer Somchai Neelahphaijit.

Violence in the south caused the imposition of martial law

It is nearly a week since the prominent Muslim lawyer and human rights activist went missing in Bangkok.

Despite initial comments by the Thai prime minister suggesting a family quarrel might have played a part in Mr Somchai's disappearance, the justice ministry has now set up a special committee to investigate.

The 52-year-old lawyer had taken on the defence of two of four Thai Muslims accused of planning attacks for the Jemaah Islamiyah network and had reportedly told colleagues of receiving threats over the case.

He was also active in the campaign for the restoration of full legal rights for Thailand's mainly Muslim south.

Efforts are under way to collect 50,000 signatures to force the authorities to lift marshal law after it was imposed because of attacks on security forces.

The prime minister says there is no evidence that the authorities were involved in the lawyer's disappearance and has warned against blaming them, but Human Rights Watch says circumstances point to the possibility that he may have been taken into custody by Thai authorities.