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The Israeli newspaper Haaretz reported Monday that Mr. Netanyahu told German Foreign Minister Guido Westerwelle that his red line would be if Iran begins enriching uranium above the 20 percent mark — the level of refinement needed to make uranium suitable for civilian energy uses.

Weapons-grade fuel typically requires enrichment to 90 percent fissile purity. But according to Haaretz, Mr. Netanyahu argued that Iran would need only six weeks to enrich from 20 percent to 90 percent.

Other analysts have said that Iran would need up to a year or more to produce weapons-grade material and fashion it into a nuclear warhead that could fit onto a missile.

Mr. Netanyahu is scheduled to address the U.N. General Assembly about Iran this month, just weeks before Election Day in the United States.

When asked whether Mr. Obama plans to meet with Mr. Netanyahu during the U.N. meeting, Mr. Carney said he didn’t have any scheduling announcements to make this far in advance.

Earlier this year, Mr. Obama sat down with Mr. Netanyahu at the White House in a show of solidarity aimed at reassuring Israelis that U.S. support for the country is undiminished.

Relations have been steadier so far this year. Mr. Obama did some fence-mending after a testy public exchange during a news conference in 2011 in which Mr. Netanyahu took issue with Mr. Obama’s calling for the creation of a Palestinian state based on boundaries that existed before the 1967 Six-Day War.

That conflict ended with Israel in control of East Jerusalem, the West Bank and Gaza.