KIEV — Defying warnings from Moscow not to confront pro-Russian militants entrenched in towns across eastern Ukraine, the interim central government in Kiev on Friday threatened to “blockade” the Kremlin’s allies in the eastern city of Slovyansk, and told Russian troops on maneuvers that any crossing of the border would be seen as an invasion.
The declarations reflected heightened worries that the government’s efforts to move against forces aligned with Moscow would trigger a Russian incursion under the guise of a humanitarian or peacekeeping initiative.
Meanwhile, Russia felt the effect of the Ukrainian crisis on its economy on Friday, when ratings agency Standard & Poor’s cut its credit grade for the first time in five years and the central bank raised interest rates to keep the sliding currency from fueling inflation.
S&P said in a statement it dropped Russia’s rating to one level above ‘‘junk status’’ because the situation in Ukraine was causing investors to pull money out of the country.
In another ominous portent, a group of foreign military observers traveling under the auspices of the Organization of Security and Cooperation in Europe, along with their Ukrainian military hosts, were detained by pro-Russia separatists in Slovyansk. It was unclear precisely how many were in the group, but the detention appeared to be the first time that members of Ukraine’s armed forces had been taken into custody by the separatists.
Unconfirmed reports earlier had identified the detainees as monitors from the OSCE in Ukraine as part of an agreement, which now appears to have all but unraveled, to ease the crisis. But OSCE officials said all their monitors had been accounted for. Tatyana Baeva, a spokeswoman for the organization in Vienna, said the reports had evidently been referring to a German-led group of eight unarmed military inspectors visiting under a separate treaty.
A pro-Russian militant leader in Slovyansk, Vyacheslav Ponomaryov, was quoted by Russia’s Interfax News Agency as saying his group was attempting to verify the identities of the detainees, who had been stopped on a bus that he said was carrying ammunition.
The events Friday morning were marked by an icy hostility between Moscow and Washington. Interim Ukrainian leaders said operations to expel pro-Russian militants in eastern cities would continue, even though military action so far has done little more than prompt Russia to stage military exercises on Ukraine’s border and raise concerns about Moscow’s next move.
“Attempts at military conflict in Ukraine will lead to a military conflict in Europe,” Ukraine’s interim prime minister, Arseniy P. Yatsenyuk, told the interim Cabinet, according to remarks broadcast live and posted on the government’s website. “The world has not yet forgotten World War II, but Russia already wants to start World War III.”
The acting head of Ukraine’s presidential administration, Serhiy Pashynskyi, said the operation to dislodge “terrorists” was continuing in and around Slovyansk and would now focus on “totally blockading” it to prevent militants from getting reinforcements and supplies. He also reported Russian military movements overnight at four locations on the Russian side of the border that involved “400 tanks, armored vehicles, and rocket launchers.”
“In the event of any crossing of the border by Russian troops, we will qualify this as an invasion and we will eliminate the invaders,” he said at a news conference in Kiev.
Ukraine’s interim defense minister, Mykhailo Koval, was quoted by the Interfax-Ukraine news agency on Friday as saying Russian troops on maneuvers had approached to within 1,100 yards of the border but had not crossed. There was no independent corroboration of the minister’s account.
Reporters in Slovyansk said there were signs of Ukrainian infantry units patrolling farmland northwest of the city, a day after Ukraine sought to dislodge pro-Russian forces from checkpoints.
On the road to Izyum, north of Slovyansk, a combined force of soldiers and Interior Ministry forces seemed to be well armed. In nearby Kramatorsk, a Ukrainian military transport helicopter was set afire, but the cause of the blaze was unclear.
In a posting on Facebook, Interior Minister Arsen Avakov said Ukraine’s military operations in the east had not been suspended, despite local news reports to the contrary.
Russia has repeatedly denied having a hand in the unrest convulsing eastern Ukraine or any intention to invade. But an announcement Thursday by Moscow that it would immediately start military maneuvers along the border with Ukraine, and a threat by Russia’s president, Vladimir Putin, of unspecified consequences for Ukraine as a result of what he called a “serious crime,” signaled a combustible new phase in a geopolitical struggle set off by the overthrow of Ukraine’s government in February.
The heightened tensions have buried already faint hopes that a deal reached April 17 in Geneva by diplomats from the European Union, Russia, Ukraine, and the United States might calm a crisis, stirring fears of a wider conflict over Ukraine, a nation of 46 million on a volatile fault line between Europe and Russia.