PALM BEACH, Fla. — The United States vowed Friday to keep the pressure on Syria after the intense nighttime wave of missile strikes from U.S. ships, despite the prospect of escalating Russian ill will that could further inflame one of the world's most vexing conflicts. Standing firm, the Trump administration signaled new sanctions would soon follow the missile attack, and the Pentagon was even probing whether Russia itself was involved in the chemical weapons assault that compelled President Donald Trump to action. The attack against a Syrian air base was the first U.S. assault against the government of President Bashar Assad.