Sheik Hamad bin Jassim al-Thani, who also serves as Qatar's foreign minister, did not elaborate on the mistakes but emphasized the mission is of "observatory nature, not to intervene in stopping the violence," CNN reported.
He said the league, which has been criticized for not stopping the violence, was seeking technical assistance from the United Nations.
Members of the opposition Syrian National Council, in a meeting Thursday with Arab League members in Cairo, requested more monitors and support, council member Walid Buni told CNN.
He called on the league to be more precise in its statements, citing an announcement by Nabil el-Araby, the Arab League secretary-general, on the release of nearly 3,500 detainees. Buni noted the information had come from the Syrian government and had not been verified.
"I think these mistakes are due to the limited experience some of the monitors have in dealing with such a regime," Buni said.
The Arab League is to meet Sunday and will discuss a preliminary report on the findings of the monitors, an Arab League official said. Syria's participation in the 22-member league was suspended in November.
Thursday, 19 people died in the violence in five provinces, the Local Coordinating Committees of Syria, an opposition activist group, said -- 12 in Deir Ezzor, three in Homs, two in Damascus province, one in Idlib and one in Aleppo.
Death tolls and other accounts could not be independently confirmed due to restrictions imposed on journalists by the Syrian government.
Meanwhile, the Free Syrian Army said it would escalate fighting to oust President Bashar Assad after accusing the Arab League of covering for the regime, rebel commander Col. Riad al-Asaad said in Turkey.
Asaad, a former Syrian air force colonel who defected last July, called the Arab League observer mission a "mockery," with "no teeth" and no influence.