MOSUL, Iraq - Iraq's indecision on whether to ask American forces to stay beyond the end of the year is pushing the U.S. close to the point where a smooth, safe troop withdrawal will be jeopardized, the top U.S. military officer said Monday as he arrived in the country.

Adm. Mike Mullen, chairman of the Joint Chiefs of Staff, said that American and Iraqi operations over the last several weeks have tamped down a recent surge in violence there, but he's not sure how long that will last.

"Now is the time. We have to know," said Mullen, speaking to reporters just before landing in Mosul.

The U.S., he said, is nearly at the point...