WASHINGTON (AFP) - The top US senator, a key White House ally, warned President Barack Obama Friday not to drop possible tax hikes on the rich from negotiations on a deal to avert a disastrous early August debt default. Democratic Senate Majority Leader Harry Reid, noting that Obama was in talks with Republican House Speaker John Boehner, declared: "I wish them well, we await their efforts" before sounding a sternly cautionary note. "I say to both the president and to the speaker here on the Senate floor, representing my Democrats and, I'm confident, many Republicans: Be very careful," said Reid.