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 US-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.”

On Wednesday, Yoon met US President Joe Biden at the White House and the United States pledged to give South Korea more insight into its nuclear planning over any conflict with North Korea, amid anxiety over Pyongyang’s growing arsenal of missiles and bombs. The two also discussed the situation in Ukraine.

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.

Answering another question, Yoon rejected the notion that the Washington Declaration he agreed with Biden meant they were accepting North Korea as a nuclear-armed state, adding that he was against treating North Korea’s possession of the weapons as a disarmament issue.

“If we were to accept nuclear weapons by North Korea, South Korea may have to possess nuclear weapons... and this would lead to a situation of disarmament. This is not something that we want to see happen,” he said.

Yoon said the Washington Declaration required Seoul to keep respecting the nuclear Non-Proliferation Treaty and not acquire its own nuclear weapons.

He said there were opinions in South Korean society that said Seoul should acquire nuclear weapons and had the technological capabilities for this, but it was complex equation about politics and economics too.

“We would need to give up many of the values that we’ve been upholding if we decide to develop our weapons,” he said. “Those opinions saying that we need to have our own nuclear arsenal are not considering all these things.”