By James Marson 

MOSCOW--Russian President Vladimir Putin blamed an escalation in violence in eastern Ukraine on the pro-Western government in Kiev, taking a hard line as U.S. and European leaders said they could tighten sanctions against Moscow unless it reins in pro-Russia militants there.

At an emergency meeting of North Atlantic Treaty Organization representatives with the Ukrainian ambassador on Monday, all 28 member countries expressed support for Ukraine, but no decisions were reached on additional action, which usually takes a meeting of higher-level officials. NATO defense ministers are meeting next week in Brussels.

"Just the meeting today...is a strong sign of the strong political support from NATO and all NATO allies for Ukraine," said Jens Stoltenberg, the group's secretary-general.

Mr. Putin, in his first comments since a rocket attack killed 30 civilians in the Ukrainian port city of Mariupol on Saturday, said on Monday that Ukraine wasn't pursuing peace and had used a cease-fire to regroup for attacks on pro-Russia forces that have declared separatist republics in the country's east.

"Unfortunately, official Kiev authorities are refusing to go down the path of peaceful resolution, don't want to seek a solution through political methods," Mr. Putin said in St. Petersburg.

The Kremlin said Monday evening that Mr. Putin spoke by telephone with French President François Hollande and German Chancellor Angela Merkel, calling for talks between the Kiev government and representatives of the separatist regions. Ukraine has in the past refused direct government talks with the militants for fear of giving them legitimacy.

Ukraine has blamed the upsurge in violence on the Russia-backed rebels, who have said they are trying to expand the territory under their control despite a peace deal signed in the Belarusian capital of Minsk in September.

Mr. Putin's comments signal further Russian defiance amid threats of new Western sanctions, which he has portrayed as an attempt to weaken Moscow.

On Monday, he accused the West of using Ukrainian forces to try to contain Russia. "In essence, it's not an army but a foreign legion, in this case a foreign NATO legion, which is of course not pursuing Ukraine's national interests. There are different aims, and they are connected with the geopolitical aim of containing Russia," Mr. Putin said.

Mr. Stoltenberg dismissed the assertion. "There is no NATO legion. The foreign forces in Ukraine are Russian. I think that is...the problem--that there are Russian forces in Ukraine, and Russia backs the separatists with equipment."

Mr. Putin has repeatedly denied accusations by the West and Ukraine that Russia is supporting rebels with weapons and troops. He described the conflict in eastern Ukraine as "civil war," and said Russia could allow Ukrainians to stay in Russia for longer than currently permitted to avoid being drafted.

The recent upsurge in violence is the most serious since the peace accord was signed. A Ukrainian security spokesman said seven servicemen had been killed and 25 injured in the 24 hours through early Monday.

Western leaders are weighing tighter sanctions against Russia unless it curbs the separatists.

The Kremlin is already under pressure from financial troubles amid sanctions and the recent plunge in price of crude oil, its main export. Standard & Poor's Ratings Services on Monday downgraded Russia's sovereign-credit rating to junk for the first time in more than a decade.

Russian Finance Minister Anton Siluanov said there was "no reason to dramatize the situation," noting that Russia would have to lose its investment-grade rating from no fewer than two agencies before foreign investors would pull out of the country's assets, Russian news agencies reported.

Russia's battered ruble plunged on the news of the downgrade in after-hours trading, bringing its slide against the dollar on Monday to 7%.

"We have more tools. I'm not today going to enumerate what the tools are, but we have more tools," U.S. Treasury Secretary Jacob Lew said in Brussels on Monday. "Our goal here is to have a diplomatic resolution where Russia lives by the Minsk accord....On the other hand, we have to be prepared to do more if Russia doesn't take that path."

German Foreign Minister Frank-Walter Steinmeier signaled he was prepared to press for new sanctions against Russia should the separatists resume their attacks.

"Of course, an attack, or certainly a broad offensive, in the direction of Mariupol and beyond would represent a qualitative change in the situation that would force us to react," Mr. Steinmeier said in Berlin on Monday after a meeting with his Latvian counterpart, Edgars Rink vičs.

Still, he noted reluctance from some European countries. "After the talks I've had in last days with some European colleagues, nobody is desperately ambitious to meet in Brussels to impose sanctions," he said.

European Union diplomats said on Monday that governments were still weighing possible further pressure against Russia and are unlikely to make any bold moves at Thursday's emergency foreign ministers meeting. Discussions will be stepped up Tuesday and Wednesday but the diplomats said there was no debate in Brussels about specific sanctions steps.

Kremlin officials warned the West it wouldn't bow to any fresh sanctions.

"Russia never assented to such threats, and such threats and blackmail never led or will lead to Russia being pressured into changing its consistent and well-known position," Kremlin spokesman Dmitry Peskov was quoted as saying by Russian news agencies.

In Donetsk, militant leader Alexander Zakharchenko said his forces were attempting to link up territory held by the two separatist republics, known as the Donetsk and Luhansk People's Republics, or DNR and LNR.

"Clashes are continuing on the whole territory of DNR. We are now trying to join with LNR, level out the front line and free up at least a brigade," Mr. Zakharchenko was quoted as saying by Russian state news agency RIA-Novosti.

In Kiev, Ukrainian Prime Minister Arseniy Yatsenyuk said the government had ordered its regions to be on high alert, which he said would allow local resources to be directed to dealing with the consequences of the upsurge in fighting.

Ukraine's Security Service posted material on its website that it said proved the rocket attack on Mariupol was carried out by a Russian-led unit, including a video statement by a man identified as the spotter for the shelling. The militants initially said they were launching an attack on Mariupol, but some later said they weren't responsible for the attack, blaming Kiev instead.

Anton Troianovski in Berlin, Nick Shchetko in Kiev, Ukraine, and Naftali Bendavid and Matthew Dalton in Brussels contributed to this article.

Write to James Marson at james.marson@wsj.com