Vladimir Putin ruled out war with Ukraine on Tuesday, but also reserved the right to use force "as a last resort" days after his forces took control of the Ukrainian peninsula of Crimea.

Breaking his silence for the first time since the revolution in Ukraine toppled Viktor Yanukovych, Putin denounced the takeover as an unconstitutional coup d'etat, insisted Yanukovych was still the legitimate head of state, although he declared him politically dead, and said he would not recognise presidential elections being held in Ukraine at the end of May.

Putin emphasised that Russia had no intention of invading Ukraine, or of annexing territory. But he also kept his options open by claiming Yanukovych had written a letter asking for Russian help.

It appeared that Putin was also seeking to send signals to the west, keen to ward off growing US-led pressure for sanctions against his regime and to sow divisions among the Europeans who are economically much more engaged in Russia than the Americans.

He also warned that sanctions were a two-way street that would affect those applying them.

The Russian leader sounded confident that Russia's takeover of the Crimean peninsula – where 16,000 pro-Russian troops are in control of the region's security and administrative infrastructure – would not descend into war.

The armed forces of Russia and Ukraine were "brothers in arms", he said. "We will not go to war with the Ukrainian people. If we do take military action, it will only be for the protection of the Ukrainian people," said Putin, adding that there was no scenario in which Russian troops would fire "on women and children".

The Russian president continued: "Ukraine is not only our closest neighbour. It is our fraternal neighbour. Our armed forces are brothers in arms, friends. They know each other personally.

"I'm sure Ukrainian and Russian military will not be on different sides of the barricades but on the same side. Unity is happening now in the Ukraine, where not a single shot has been fired, except in occasional scuffles."

Putin denied that the Russian-speaking soldiers occupying key Crimean military sites were Russian special forces, saying they were pro-Russian local self-defence forces.

"There are many military uniforms. Go into any local shop and you can find one," he said.

As Putin was speaking a standoff between Russian-speaking and Ukrainian soldiers at the Belbek airbase in Crimea threatened to erupt into clashes. Meanwhile, the US secretary of state, John Kerry, arrived in Kiev to hold crisis talks with the new Ukrainian government, hours after Washington suspended all military engagements with Russia, including exercises and port visits, in response to Moscow sending troops into Crimea.

Kerry pledged $1bn (£600m) in loan guarantees as a show of support for the new government in Kiev.

Responding to journalists' questions on the US position and Kerry's involvement, Putin said: "The secretary of state is important but he does not determine the policy of the United States. I am hearing many different things."

Barack Obama had warned that the US government would look at economic and diplomatic sanctions that would isolate Moscow over Russia's involvement in the Ukraine crisis. The US also put on hold trade and investment talks with Russia .

Referring to these threats from the White House and other western leaders to eject Russia from the G8 ahead of looming talks, the president said that he was still preparing to host a G8 summit in Sochi in June, but added: "If the leaders don't want to come, fair enough."

During his impromptu and occasionally rambling remarks, Putin said he had advised Yanukovych not to dismiss the feared Ukrainian riot police from Kiev, warning the ousted leader that "chaos, anarchy" would ensue if he did. "Yanukovych has no political future now, I have told him that," Putin said.

Earlier on Tuesday, the Kremlin ordered Russian troops carrying out exercises near Ukraine's eastern border to return to their barracks.

Russia's invasion of Ukraine's Crimean region has so far been bloodless despite tense standoffs and threats from both sides.

The majority of the Belbek airbase has been taken over by heavily armed Russian troops, including several armoured vehicles with Russian military plates. They have been joined by half a dozen masked men, who identified themselves as the self-defence unit of Sevastopol.

One of them, who said he was an entrepreneur named Ura, was dressed in military fatigues. "I don't want my city to be bombed by the planes in this airfield if Kiev fascists give the order. That's why we're here," he said.

Asked about the Russian troops he said, "I don't know what you're talking about. Can you see any identifying marks on their uniforms? I can't. They're not Russian troops."

Asked about the military vehicles with Russian plates, he said: "I don't know; they're probably left over here from before."

The Ukrainian regiment who had been stationed at Belbek and are still in control of some of the buildings at the base, say they want to be able to guard the weapons storage facility. Yuli Mamchur, the commander, told the Guardian: "I just want my men to be able to do their duty."

He had given the Russians a 2pm (local time) deadline to allow them to enter the weapons store with Russian troops. After a brief standoff the Ukrainian troops retreated. It was not clear whether they planned to return.

Earlier, a supposed Russian ultimatum for two Ukrainian warships to surrender passed without action from either side on Tuesday morning.

The warships remained anchored in the Crimean port of Sevastopol early on Tuesday, a day after Ukrainian authorities claimed that Russian forces had issued the ultimatum for the ships to surrender or be seized. Russian defence ministry spokesman Vladimir Anikin said on Monday that no ultimatum had been issued.

Russia's ambassador to the UN claimed on Monday that the fugitive Ukrainian president had requested that Russian soldiers be sent to Crimea "to establish legitimacy, peace, law and order". Vitaly Churkin told a UN security council meeting that Yanukovych wrote to Putin on Saturday.

France's ambassador to the UN, Gérard Araud, said the letter was just a piece of paper handed to Yanukovych that "got his signature". Asked if the letter was phoney, Araud replied: "It's not a false letter, it's a false president."