UNITED NATIONS (Reuters) -- Israel warned the UN Security Council on Monday that it could not be expected to "stand idle" as Syria's civil war spills over its border, while Russia accused armed groups of undermining security between the states by fighting in a demilitarized zone.

Israeli UN Ambassador Ron Prosor wrote to the 15-member council to complain about shells from Syria landing in Israel.

"Israel cannot be expected to stand idle as the lives of its citizens are being put at risk by the Syrian government's reckless actions," Proser wrote. "Israel has shown maximum restraint thus far."

Israel does not have a reputation for being idle. Israeli Defense Minister Ehud Barak said that an attack on a Syrian arms complex on Jan. 30 showed Israel was serious about preventing the flow of heavy arms into Lebanon, appearing to acknowledge that Israel carried out the strike.

The United Nations says more than 70,000 people have been killed during a two-year revolt against Syrian President Bashar Assad, which began as peaceful protests but turned violent when Assad's forces cracked down on the demonstrations.

With nearly 1 million Syrian refugees flooding neighboring Turkey, Jordan, Iraq and Lebanon as the conflict worsens, the United Nations has warned that the fighting has developed sectarian overtones and could engulf the region.

Russia's UN Ambassador Vitaly Churkin, president of the Security Council for March, said the security situation between Syria and Israel was also being threatened by "a very new and dangerous phenomenon" of armed groups operating in a so-called area of separation in the Golan Heights between the countries.

Israel has occupied the Syrian Golan Heights since 1967. Syrian troops are not allowed in the area of separation under a 1973 ceasefire formalized in 1974. Israel and Syria are still technically at war. The area is patrolled by UN peacekeepers.

"It's something which potentially can undermine security between Syria and Israel," Churkin told reporters, adding that the UN peacekeeping force, known as UNDOF, was unable to cope with the situation.

"Unfortunately there is nothing in the UNDOF mandate that allows them or equips them to deal with that situation because they are unarmed observers," Churkin said.

Croatia's government said on Thursday that it planned to pull out of UNDOF as a precautionary step following media reports that Croatian arms were being sent to Syrian rebels fighting Assad. Croatia has 98 troops in the 1,000-strong force.

The UN peacekeeping department is attempting to find replacements for the Croatians but it will not be easy given the tension in the region, UN officials told Reuters on condition of anonymity.

The UN Security Council has been deadlocked on Syria since 2011 over Russian and Chinese refusal to consider sanctions against Assad's government. They have vetoed three resolutions condemning Assad's crackdown on the opposition groups.