SIMFEROPOL, Ukraine — Ukraine’s security chief ordered a full call-up of reserve troops Sunday morning, as Russian troops took control of the Crimea peninsula and the bloodless clash threatened to explode into fighting.

Andriy Parubiy told reporters that the council had ordered the defense ministry to “call on all those that armed forces need at the moment across Ukraine,” adding that the mobilization was to “ensure the security and territorial integrity of Ukraine”.

Earlier in the day, a convoy of hundreds of Russian troops were spotted heading toward the regional capital of Simferopol.

Russian troops took over the strategic Black Sea peninsula on Saturday without firing a shot and the new government in Kiev has been powerless to react.

On the road from Sevastopol, the Crimean port where Russia maintains a naval base, to Simferopol on Sunday morning, AP journalists saw 12 military trucks carrying troops, a Tiger vehicle armed with a machine gun and also two ambulances.

Ukraine’s acting president, Oleksandr Turchynov, announced late Saturday that he had ordered Ukraine’s armed forces to be at full readiness because of the threat of “potential aggression.” He also said he had ordered stepped-up security at nuclear power plants, airports and other strategic infrastructure.

On Crimea, however, Ukrainian troops have offered no resistance.

Russian military forces operating in Crimea were reported to have taken weapons from two Ukrainian military posts, according to a Ukrainian Defense Ministry sources cited by the Interfax news agency. The Russian troops were said to have urged military personnel at the posts to side with the “legitimate” leaders of the Crimean Peninsula.

The new government came to power last week following months of pro-democracy protests against the now-fugitive president, Viktor Yanukovych, and his decision to turn Ukraine toward Russia, its longtime patron, instead of the European Union.

Ukraine’s population of 46 million is divided in loyalties between Russia and Europe, with much of western Ukraine advocating closer ties with the EU, while eastern and southern regions look to Russia for support. Crimea, a semi-autonomous region that Russia gave to Ukraine in the 1950s, is mainly Russian-speaking.

The Russian move into the Crimean peninsula Saturday led to international calls for Moscow to pull back its troops.

On Saturday, Ukraine’s ambassador to the United Nations asked an emergency session of the Security Council “to do everything possible now” to stop Russia’s “aggression” as its troops took over the strategic Crimea region.

But action by the UN’s most powerful body appears unlikely. As a permanent member, Russia has veto power and can block the council from adopting any resolution criticizing or sanctioning Moscow.

UN Secretary-General Ban Ki-moon urged Russian President Vladimir Putin in a phone call to “urgently engage in direct dialogue with the authorities” in Kiev.

However, Putin threatened that if there is an escalation of violence against Russian speakers in eastern Ukraine and Crimea, Russia “will take necessary measures in [the] framework of international law,” Interfax reported.

Calling the situation in Ukraine “as dangerous as it is destabilizing,” US ambassador to the UN Samantha Power told the council, “It is time for the Russian military intervention in Ukraine to end.”

Power and other members of the council called for sending international monitors to Ukraine as soon as possible to observe the situation, and Power warned that “Russia’s provocative actions could easily push the situation beyond the breaking point.” She also mentioned work on an international mediation mission to send to Ukraine.

The Security Council met in emergency session for the second straight day on the rapidly developing events in Ukraine. It even met briefly in an open, televised session, despite objections from Russia, then resumed meeting behind closed doors.

Current council president, Luxembourg Ambassador Sylvie Lucas, said members stressed the importance of Ukraine’s territorial integrity and the need to lower tensions, in addition to the need for international monitors.

Russia’s UN ambassador, Vitaly Churkin, said the new government in Kiev needs to get away from “radicals” and warned that “such actions they’re taking could lead to very difficult developments, which the Russian Federation is trying to avoid.”

Churkin also accused the West of interfering in the recent Kiev demonstrations that turned violent amid tensions over the decision by Ukraine’s now-fugitive president, Viktor Yanukovych, to turn Ukraine toward Russia, its longtime patron, instead of the European Union.

Russia has given refuge to Yanukovych, who fled a week ago.

Churkin said Russia was intervening at the request of pro-Russian authorities in the semi-autonomous Crimea, which is largely Russian-speaking and home to Russia’s Black Sea navy fleet.

British Ambassador Mark Lyall Grant, who called for Saturday’s meeting, told reporters afterward that “there is no justification for Russia’s military activities in the last 48 hours.”

Deputy Secretary-General Jan Eliasson called the situation in Ukraine “very difficult and very dangerous” and said they were seeing “negative signs, serious signs, risks of escalation.”

Ukraine’s UN ambassador, Yuriy Sergeyev, asked the other four permanent Security Council members — the US, Britain, France and China — for help, adding that Russia had rejected Ukraine’s proposal to hold immediate bilateral consultations.

When asked later whether Ukraine is at war with Russia, Sergeyev said, “No. We are not at war. We are trying to avoid any clashes. We are being provoked.”

Ban, the UN chief, said earlier Saturday that he is “gravely concerned about the deterioration of the situation” in Ukraine and called for “full respect for and preservation of the independence, sovereignty and territorial integrity” of the country.

Ban later spoke by telephone with Putin, and a statement from Ban’s office said, “It is crucial to restore calm and proceed to an immediate de-escalation of the situation.”

“Cool heads must prevail, and dialogue must be the only tool in ending this crisis,” Ban said.

Ban planned to meet Sunday in Geneva with his special envoy Robert Serry, the Netherlands’ first ambassador to Ukraine.

Ban on Friday asked Serry to go to Crimea as part of a fact-finding mission. However, after consulting with authorities in the region, Serry decided that a visit was not possible.

Lyall Grant said his understanding was that Serry couldn’t go because airspace above Crimea has been closed. Eliasson called the decision “purely logistical.”

AFP contributed to this report