WASHINGTON, April 28 (Reuters) - South Korean President Yoon Suk Yeol said on Friday it was necessary to ensure Russia’s invasion of Ukraine does not succeed and that Seoul was considering its options when it came to lethal aid to Kyiv.

In a speech at Harvard University's Kennedy School on the fifth day of a state visit to mark the 70th anniversary of the U.S.-South Korean alliance, Yoon said the Russian invasion was a violation of international law and the rights of Ukrainians.

"We should prove that such attempts will never reach success, to block further attempts being made in the future," he said, according to simultaneous translations of his remarks.

Yoon was asked about the possibility of South Korea providing lethal aid to Ukraine, and replied:

"We are closely monitoring the situation that's going on the battlefield in Ukraine and will take proper measures in order to uphold the international norms and international law. "Right now we are closely monitoring the situation and we are considering various options."

Yoon told Reuters in an interview last week before leaving for the United States that Seoul might extend its support for Ukraine beyond humanitarian and economic aid if it comes under a large-scale civilian attack, signaling a shift in his stance against arming Ukraine for the first time.