Turkey's prime minister said Tuesday that Syria's president must step down over the country's crackdown on dissent, ratcheting up the pressure on the increasingly isolated Bashar Assad, as Syrian activists reported that four children were killed by security forces. The Local Coordination Committees, a key activist network, and the British-based Syrian Observatory for Human Rights, said the children, between the ages of 10 and 15, were killed by gunshots fired at random from a military checkpoint near the town of Houla in the restive Homs province. The LCC said Syrian forces backed by tanks and armored vehicles stormed the area of Houla and were besieging the district of Bayada in Homs, a hotbed of dissent against President Assad's regime.