WASHINGTON, Oct 31 (Reuters)

U.S. national security adviser John Bolton said on Wednesday the Trump administration wants sanctions on Iran's crude exports to strain Tehran, but does not want to harm countries that depend on the oil.

"We want to achieve maximum pressure but we don't want to harm friends and allies either," Bolton said in a talk at the Hamilton Society. Bolton said the administration understands that a number of countries, some close geographically to Iran which he visited last week, and others "may not be able to go all the way, all the way to zero immediately."

It was a more conciliatory tone about the sanctions from Bolton, a proponent of being tough on Iran and winding down its crude exports to zero. Still, Bolton said that consequences can already be seen in Iran including the collapse of the rial, its currency.

"I think it's important that we not relax in the effort," he said. The administration is weighing whether to give waivers to some countries that have cut their purchases of Iranian oil on Nov. 5, when sanctions on Iranian exports snap back. Three of Iran's five largest buyers of crude - China, India and Turkey - have resisted calls by Washington to end their oil purchases outright.

This week South Korea asked Secretary of State Mike Pompeo for "maximum flexibility" on its request for a waiver to prevent companies there from being hit by the sanctions. Other countries, including Iraq and Afghanistan, depend on some imports from Iran. The administration has said it is considering waivers on a case-by-case basis.

(Reporting by Steve Holland, Timothy Gardner)