The UN, appalled at the ethnic slaughter going on in Sudan (in the south and the west/Darfur) has put an embargo on weapons shipments to Sudan. Everyone agrees this is a splendid idea, and agrees to abide. Yet Sudan, and various rebel groups, continue to get weapons, and other military equipment. And it's no secret who's supplying the stuff. China ships weapons to the Sudanese government, and denies it. Ukraine appears to be the main supplier of weapons to Sudanese rebels in western and southern Sudan. These are shipped in through Eritrea (for Darfur) and Kenya (for southern Sudan.)
It was two years ago that the UN discovered Kenya was importing large quantities of weapons (at least 77 tanks, 15 jet fighters and 40,000 assault rifles and machine-guns in the last year or so) without reporting them. A 1991 international treaty, which Kenya signed, obliges all nations to report weapons exports and imports (the better to control the illegal trade in arms.) Not everyone follows the rules.
This particular scandal came has come out a 2008 pirate attack that seized a Ukrainian ship while it was passing through the Gulf of Aden, on its way to Kenya, carrying 33 T-72 tanks and tons of smaller weapons. The UN had no record of this transaction (Kenya admitted the weapons were theirs.) Most of these unreported weapons for Kenya ended up with the south Sudan rebels.
The UN is also particularly concerned about trying to limit the undocumented sale of small arms. Most of these weapons are of Russian design (although manufactured by several countries, mainly Russia and China). The most common weapon is the AK-47 (and its many variants). "Small arms" include machine-guns of 7.62mm, and smaller, caliber, as well as pistols and machine pistols. The international trade in small arms is estimated at $4 billion a year, and about a quarter of that is illegal. It's believed that two thirds, or more, of the combat deaths each year are from small arms. This is particularly true in wars employing many irregular troops. Traditional tribal conflicts in Africa and Asia have become a lot more bloody because of the proliferation of small arms, usually illegally obtained ones.
The UN wants to impose more regulations on legitimate small arms sales, and encourage more vigorous prosecution of illegal arms traders. This effort, like an earlier one that banned the use of anti-personnel mines, would largely be symbolic, a feel-good measure for those pushing it. The reality is that the current proliferation of small arms is largely a result of the end of the Cold War. The former communist countries found themselves with millions of AK-47s and light machine-guns, as well as RPGs, landmines, grenades and ammo they no longer needed. Ukraine, then a part of the Soviet Union, inherited over seven million Soviet AK-47s and machine-guns, when it became independent from the Soviet Union in 1991.
Communist countries were police states with very large armies and police forces. Most of these personnel were armed with AK-47s, although the majority of the troops were reservists. So their weapons spent most of the time locked up in armories. Since the 1990s, these armories were either looted (as in Albania and Iraq), or had their contents sold off, by corrupt officials, in illegal arms deals. China still manufactures a lot of AK-47s, and is willing to sell them to shady dealers. The AK-47s have flooded Africa, Asia and the Middle East in the last fifteen years, making them very cheap (less than $100).
The major gunrunners are known, but manage to find sanctuary in Eastern Europe and Russia. Another major source of such weapons are corrupt officials, who sell off weapons to anyone. Such corrupt officials also sell older weapons, instead of following orders and destroying them. An additional international treaty would not stop the gun runners or corrupt officials. Many nations that signed the 1991 treaty have not reported all their exports and imports. Kenya and China are just two of many offenders. In 2006, Italian police arrested some local gangsters and found that they were brokering an unregistered sale of half a million Chinese AK-47s to Libya.
Many countries don't want to call out Ukraine for arming the Sudanese rebels, because the world is appalled at the brutality with which Sudan treats its ethnic minorities. That's where the rebels come from in Sudan, and it's hard to get too worked up about anyone who is arming the rebels.
In practice, the key to slowing the trade in small arms is local action. This is much more difficult than enacting a new arms control treaty. Such treaties are nothing new. For most of the last thousand years, the Roman Catholic church has periodically tried to ban some weapons, and warfare in general. But weapons control, like politics, is all about local situations. There is no easy solution.