In his harshest comments to date on the situation in Libya, Prime Minister Recep Tayyip Erdogan of Turkey, a key regional power broker, told reporters in Istanbul on Tuesday that Col. Muammar el-Qaddafi must immediately step down. Turkey has historic and business ties to Libya and during the more than two-month long conflict has tried to act as an intermediary between Colonel Qaddafis government and rebels seeking his ouster. As a result, each side has accused Turkey of favoring the other, or of hedging its bets. But on Tuesday, Mr. Erdogan appeared to draw a line, telling a news conference that Colonel Qaddafi had violently resisted calls for change and must leave power for the good of the country and the Libyan people. His strong words came a day after Turkey closed its embassy in the Libyan capital, Tripoli, apparently worried about the angry crowds that had attacked the missions of countries from the NATO alliance, of which Turkey is a member.