Republican U.S. presidential candidate Ben Carson canceled planned trips to Israel and Africa due to what his campaign on Thursday called significant security concerns.

Campaign spokesman Doug Watts confirmed that the trip, scheduled for after the Christmas holiday next week, has been canceled, The Hill reported. The campaign did not offer details about the nature of the security concerns, but Watts called them “significant.”

“They were of such a nature that we found it prudent not to put anyone at any kind of risk,” he added.

Carson was scheduled to travel to Israel, Kenya, Zambia and Nigeria from December 27 to January.

Earlier this month, Carson, who has been criticized for a lack of foreign policy experience, traveled to Jordan to visit a camp for Syrian refugees.

The retired pediatric neurosurgeon is seeking to bolster his foreign policy bona fides to convince voters that he’s a qualified commander in chief after stumbling on national security matters.

The GOP presidential race has focused heavily on national security after terror attacks in Paris and San Bernardino, California, and Carson has acknowledged that questions over his foreign policy acumen have weighed on him.

After jockeying to the top of the GOP primary polls alongside Donald Trump for much of November, Carson's support among Republican voters has declined in recent weeks, according to opinion polls. The trips abroad were intended to address concerns that he could not handle international diplomacy.