WASHINGTON—Sen. John McCain (R-Ariz.) secretly traveled to northern Syria last weekend to speak with American military officials and Kurdish fighters at the forefront of the push to drive Islamic State out of their de facto capital of Raqqa, according to U.S. officials.

The unusual visit, which officials said was organized with the help of the U.S. military, came as the Trump administration is debating plans for an accelerated military campaign against Islamic State, also known by the acronyms ISIS and ISIL.

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