Israel secured supplies of ammunition from the Pentagon last month without the approval of the White House or the State Department, The Wall Street Journal reported Thursday.
President Barack Obama's administration, caught off guard as it tried to restrain Israel's campaign in Gaza, has since tightened controls on arms shipments to Israel, the newspaper said, quoting US and Israeli officials.
The newspaper said Obama and Israeli Prime Minister Benjamin Netanyahu had a particularly tense phone call on Wednesday and that the Israeli leader wanted US security assurances in return for a long-term deal with Hamas.
But the newspaper said Netanyahu had essentially "pushed the administration aside," reducing US officials to bystanders instead of their usual role as mediators.
State Department spokeswoman Marie Harf disputed the article. She acknowledged that the administration was looking carefully at arms shipments to Israel but said the process was "by no means unusual."
"Given the crisis in Gaza, it's natural that agencies take additional care to review deliveries," she told reporters.
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"The United States has an unshakable commitment to Israel's security. No country has done more to support their security than the United States," she said.
Harf declined to discuss specific arms shipments but noted that Obama last week signed a bill authorizing another $225 million for Iron Dome -- the US-backed missile defense system that has sharply curtailed Israeli casualties in the latest conflict.
A Pentagon spokesman said there was an "existing" procedure for handling arm sales to Israel.
"It's a process that we're constantly looking at, constantly assessing," he said.
"Certainly in light of the much increased operational tempo that the Israeli Defense Force is under now, as they defend themselves from Hamas, it warrants -- that process continues to warrant assessment and review."
Obama has defended Israel's use of force against rockets fired by Hamas from the Gaza Strip but has repeatedly voiced concern over the number of civilian casualties and called for a settlement that gives more hope to Palestinians.
Israel and militants in Gaza were holding their fire Thursday after a new truce in the conflict that has killed 1,962 Palestinians and 67 people on the Israeli side since July 8.