WASHINGTON — The United States has increased the number of Marines and aircraft stationed in Sicily who could be called upon to evacuate Americans from the U.S. embassy in Tripoli as unrest in Libya grows, two U.S. officials said on Monday.About 60 more Marines and another four Osprey aircraft, whose tilt-rotar engines allow it to land like a helicopter but fly like an airplane, were being sent to Naval Air Station Sigonella in Sicily from their base in Spain.That brings the total number of Marines stationed as a precaution in Sicily to around 250, the two officials said, speaking on condition of anonymity. Their location in Italy moves them closer to Libya, speeding response times.One U.S. official added that forces at Sigonella were on heightened alert.Heavily armed gunmen stormed Libya's parliament on Sunday and gunfire erupted across Tripoli, where rival militias clashed in some of the worst violence in the city since the end of the 2011 war against Muammar Gaddafi.