ST. PETERSBURG, Russia -- Russian President Vladimir Putin met with top editors of international news agencies Friday, saying he would observe constitutional term limits that would prevent him from running for a new term in 2024. However, some observers have suggested the Russian leader might seek to have the constitution changed.

On tensions with Britain over allegations that Russia was behind the March poisoning of a Russian former spy in Britain, Putin said there should "either be a joint, full-value, objective investigation or simply stop talking about this subject because it doesn't lead to anything except worsening relations."

Russia has repeatedly demanded that Britain let it take part in investigating the case.

Earlier in the day, Putin spoke at a business forum and commented on the U.S. exit from the Iranian nuclear deal. He said the move could trigger dangerous instability and raise new threats for Israel if Tehran resumes a full-fledged nuclear program.

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"We can't sort things out with North Korea. Do we want another problem on the same scale?" Putin asked.

Putin said the U.S. withdrawal from the 2015 agreement came even as the international nuclear watchdog confirmed that Tehran was fulfilling its obligations. "What should it be punished for, then?" Putin asked.

President Trump's administration has demanded that Iran stop the enrichment of uranium and end its involvement in Syria, Yemen, Lebanon and Afghanistan in order to negotiate a new deal.

"If international agreements are revised every four years it would offer zero horizon for planning," Putin said. "It will create the atmosphere of nervousness and lack of trust."

While Israel hailed the U.S. withdrawal, Putin warned that the move could eventually hurt Israeli security if the deal completely falls apart.

"Would it be better for Israel if Iran opts out of the deal or is pushed out of it?" he asked. "In that case, its nuclear activities would become totally non-transparent. What kind of risks will it entail?"

The Iran deal was the first time Russia, France, Germany and others had agreed on a major international issue since relations between Russia and the West chilled over Moscow's annexation of Crimea in 2014.

Moscow's hopes for better ties with Washington under Mr. Trump have fizzled amid investigations into alleged collusion between Mr. Trump's campaign and Russia. Speaking at the panel, Putin again denied any meddling in the U.S. election in 2016.

While criticizing the U.S. policy on Iran, Putin had some warm words for Mr. Trump and held out hope for holding a summit with him.

"We certainly can't be happy with the level and nature of Russia-U.S. relations," Putin said. "We are ready for dialogue. It long has become overdue."

He also suggested that Mr. Trump might have won a few points domestically by exiting the Iran deal. "He fulfilled his campaign promises and in that sense he might have won in terms of domestic politics," Putin said.

The Russian leader also engaged in a tongue-in-cheek exchange with French President Emmanuel Macron, saying with a smile that Russia could help protect Europe if its rift with the U.S. widens over Iran.

"Don't you worry, we will help ensure your security," Putin said. Macron responded on a serious note that France and its allies could stand for themselves.

Russian President Vladimir Putin and his French counterpart Emmanuel Macron attend a session of the St. Petersburg International Economic Forum (SPIEF), Russia May 25, 2018. Dmitri Lovetsky/Pool via REUTERS

In his speech at the forum and during talks with Putin on Thursday, Macron called for closer ties between France and Russia despite their differences.

Japanese Prime Minister Shinzo Abe also spoke at the forum and called for closer cooperation with Russia.

The presence of Macron and Abe and their statements in favor of cooperation with Moscow were important for Putin, indicating that the U.S.-led efforts to isolate Russia face increasing obstacles.

The U.S. and its allies have hit Russia with several waves of sanctions that badly hurt its economy.

Putin sharply criticized the sanctions, saying they signal "not just erosion but the dismantling of a system of multilateral cooperation that took decades to build."