South Korean shares closed slightly lower on Monday on losses made by heavyweight technology stocks Samsung Electronics (KRX:005930) and SK Hynix (KRX:000660), shares of which fell by nearly 5% and 2%, respectively, at market close.

Rising oil prices also put a strain on investor optimism amid the continued hostilities between the U.S. and Iran. Brent crude for August delivery traded at $73.21 a barrel as of 03:30 GMT, rising $1.27 higher from its level before the U.S. and Israel launched military action against Iran on Feb. 28.

The benchmark Korea Composite Stock Price Index, or Kospi, marginally declined by 0.2%, or 16.56 points, to close at 8,394.65. The Kosdaq, however, rose by 8.1%, or 69.2 points, to end at 920.57.

On the corporate front, the labor union of Samsung Biologics (KRX:207940) on Sunday voted to exit the Samsung Group federation, with an aim to alter the union's organizational structure, Pulse News reported Monday.

The union conducted a vote from June 24 to June 28, with 2,392 of 2,479 participating members, or 96.5%, backing the proposal to quit the group federation. The proposal required a majority turnout and approval from at least two-thirds of participating members to pass, according to the report.

Responding to MT Newswires, Samsung Biologics said the union's withdrawal from the super‑union is expected to have minimal impact, adding discussions on scheduling proposals remain ongoing, and it remains committed to operational stability and minimizing any effect on client projects.

Shares of the company jumped nearly 8% at market close.

In other news, HD Korea Shipbuilding & Offshore Engineering (KRX:009540) won an order for two liquefied petroleum gas (LPG) carriers from an Asia-based shipper, according to a Monday filing with the Korean Exchange.

The deal is valued at 366 billion won. The shipbuilder will deliver the vessels by Dec. 31, 2029.

Shares of HD Korea Shipbuilding rose over 6% at market close.