Al-Zawahiri said: "Hamas has sold out so it can keep a hold of a third of government".
"But what government? A government that doesn't even have the right to enter or leave [the Palestinian territories] without Israeli permission."
He said: "The leadership of Hamas government has committed an aggression against the rights of the Islamic nation by accepting what it called ... respecting international agreements.
"I am sorry to have to offer the Islamic nation my condolences for the [virtual demise] of the Hamas leadership as it has fallen in the quagmire of surrender."
But Hamas's Osama noted that the Mecca agreement was supported by most of the respected leaders in the Muslim world and would not be undermined by a comment from anyone who was not aware of all the facts and political developments.
"If anyone wishes to express reservations on the Mecca agreement, we respect that but at the same time we affirm that Hamas would never accept to be dealt with as if the movement is not rational and needs guidance from someone."
"Hamas has managed to withstand an unfair blockade for a year now. This blockade on Hamas could have been lifted if Hamas agreed to recognise the Zionist entity [Israel], but Hamas will never recognise Israel," he added.
|Fresh violence erupted in Gaza before the talks between Abbas and Olmert in Jerusalem [Reuters]|
Hamas, which came to power in January 2006, refuses to recognise Israel - a stance which has led to Western countries boycotting the Hamas-led Palestinian government after its electoral win in January 2005.
Al-Zawahiri's and Osama's comments came as Ehud Olmert, the Israeli prime minister, and Mahmoud Abbas, the Palestinian president and head of Fatah, were reported to have made little progress during talks in Jerusalem.
The two men met in Olmert's official residence on Sunday hours after violent clashes took place between Hamas and Fatah fighters in Gaza.
Olmert has already said he will boycott any unity government that fails to recognise Israel's right to exist.
But he also promised to keep channels of communication open and officials said more talks between the two men were likely to take place.
Olmert and Abbas last met on February 19 in trilateral talks with Condoleezza Rice, the US secretary of state, that ended with no sign of progress towards resuming peace talks that ended six years ago.
The unity deal, reached last month in the Saudi city of Mecca, only agrees to "respect" past agreements, falling short of international conditions.
"The Israeli government has expressed its disappointment over the Mecca accord and has not changed its position," Olmert told his cabinet before the summit on Sunday.
"We hope that the Palestinian government that is to be formed will accept the Quartet principles," he said, referring to the group of Middle East mediators comprising the UN, US, EU and Russia.