A former senior U.S. defense official viewed as a possible successor to Leon Panetta as defense secretary said the Philippines has recently mistaken U.S. renewed engagement in the region as an opportunity to more assertively pursue territorial claims against China.

Michèle Flournoy, who served as undersecretary for defense policy until February 2012, said last month while the U.S. needed to send clear signals of support for its allies in the region, it also needed to ensure that support didn’t lead allies to act provocatively.

Naming the Philippines specifically, she said there was a risk of Manila “mistaking U.S. support for an opportunity to be much more assertive in staking their claims. I think we have to be careful that we don’t feed that dynamic.”

The comments by Ms. Flournoy came during a question-and-answer session at Australian National University on Nov. 29. Video of the remarks was posted on YouTube this week.

“I do think there is a danger of some of our friends occasionally misreading, or miscalculating, in terms of the support that they have from the United States,” Ms. Flournoy added.

Tensions between China and the Philippines escalated rapidly this year over longstanding territorial disputes in the South China Sea. Many feared a prolonged spring standoff between Chinese and Philippine government vessels in the sea’s disputed Scarborough Shoal would trigger conflict.

The standoff eventually drew down peacefully, but revived questions over what the U.S. would do in the event of an armed China-Philippines clash. The U.S. is obligated to protect the Philippines by a 1951 mutual-defense pact. It remains unclear, however, whether a conflict in disputed territory would trigger the U.S. to act in defense of its Asian ally.

The U.S. has repeatedly said it doesn’t take sides in territorial disputes, a point Ms. Flournoy reiterated during her Australia comments.

The sea is claimed in whole or in part by China, Taiwan, the Philippines, Vietnam, Malaysia and Brunei. It’s home to critical shipping lanes and is also believed to hold rich oil-and-gas deposits. Estimates on resource deposits vary widely, anywhere from 28 billion barrels of oil or less to as much as 213 billion barrels, according to some accounts. The ongoing tensions, however, have stunted resource development in the South China Sea.

–Brian Spegele. Follow him on Twitter @bspegele.