Reporting from Washington — The Obama administration is increasingly concerned about how long Iran may take to respond to its peace overture, and worried that delays will build pressure at home and abroad that could undermine the effort to overcome decades of antagonism through face-to-face talks.

In March, President Obama videotaped an appeal addressing Iranian leaders [and people]. But U.S. officials now worry that Tehran, currently engaged in a presidential election campaign, may not provide an answer until autumn or even the end of the year.

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