President Donald Trump's top adviser on Ukraine has testified about the president's July phone call to his Ukrainian counterpart.

Alexander Vindman said that he raised concerns twice about the president's efforts to pressure Ukraine to investigate political rival Joe Biden.

He is the first witness who actually sat in on that call, which has led to bipartisan calls for the president to be impeached.

Al Jazeera's Heidi Zhou-Castro reports from Washington, DC.

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