John Kerry made a surprise visit to Iraq on Sunday, his first such trip since he became secretary of state, following in the footsteps of Hillary Clinton.
Kerry told reporters he encouraged Iraqi prime minister Nuri al-Maliki to help stop Iranian arms flights over its air space to Syria, which the US suspects are taking place almost daily.
"Anything that supports President Assad is problematic," Kerry said, according to Reuters. "I made it very clear to the prime minister that the overflights from Iran ... are in fact helping to sustain President Assad and his regime."
The Obama administration has been unable to persuade Iraq to block such flights or even perform regular inspections.
Iraq claims the flights carry only humitarian aide, but the US claims the number of flights and overland vehicle traffic crossing Iraq points to regular arms shipments.
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Kerry's visit to Iraq was the first by a US secretary of state since 2009.
He also addressed recent Sunni protests that could potentially lead to increased leeway for extremist groups such as Al Qaeda to operate.
Kerry met with al-Maliki and parliament speaker Osama al-Nujaifi, and asked the current Iraqi Shiite-led government to work more closely with Iraqi Sunnis, who have demonstrated increasing dissatisfaction with the current arrangement.
The secretary of state also had a phone conversation with Massoud Barzani, the president of the autonomous and long-time contested region of Kurdistan, according to the BBC.