The United States could hit Syria with three days of missile strikes, perhaps beginning Thursday, in an attack meant more to send a message to the Syrian regime than to cripple its military, senior U.S. officials told NBC News.

The disclosure added to a growing drumbeat around the world for military action against Syria, believed to have used chemical weapons in recent days against scores of civilians and rebels who have been fighting the government for two years.

In three days of strikes, the Pentagon could assess the effectiveness of the first wave and target what was missed in further rounds, the senior officials said.