ISTANBUL — Turkey won a measure of international support in its increasingly tense standoff with the United States on Wednesday when Qatar offered a relatively small but symbolically important financial lifeline.

Qatar pledged to invest $15 billion in Turkey after a lunch in Ankara, Turkey’s capital, between President Recep Tayyip Erdogan and Sheikh Tamim bin Hamad al-Thani, the emir of Qatar, representatives of both governments said.

The sum is a small fraction of what Turkey would need to shore up its faltering economy or pay its dollar debts, which have become increasing unsustainable after a sharp decline in the Turkish lira.

But the promise of investment was trumpeted by the Turkish news media as a victory for Mr. Erdogan on the same day that Turkey rejected a second legal appeal to release the American pastor Andrew Brunson. Turkey also sharply raised tariffs on American goods on Wednesday, as it pushed back against pressure from the United States.